Disagree, but not wholly. I would actually group Gen 3, 4, and 5 into one category while putting 6 and 7 together. 8 is a category all to itself imo.
4 is very much a continuation of 3 in terms of gameplay and design choices. You see this largely in things like story and gameplay elements being echoed across the titles. I've always thought that DPP was pretty much a spiritual sequel to RSE; when you strip them down to the essentials, they basically are the same story, and DPP builds on a lot of the concepts RSE introduced. But despite 5 being a stated reset for the series, I don't view it as mechanically or conceptually that different. Sure there's some needed QoL changes (Pokemon centres being merged with marts, infinite TMs) but they're mostly skindeep changes that enhance gameplay rather than fundamentally alter it. This is partly to do with 5 still being on the DS, but there are so many things which stay intact from 3 all the way up to 5 that I don't think they can be said to be a different category.
6 is a whole new ballpark and it's nothing like Gen 5 imo. It introduced all sorts of new concepts like Super Training; GF's stated intention here was to make the EV system more open and transparent and this is where that starts. It's also noticeably where the difficulty lowers, and tweaks like the fact that you can do one battle at the Maison at a time, rather than the fixed rounds of seven all the previous games enforced, make the gameplay of Gen 6 a radically different experience than that of earlier gens. The PSS, too, means that gameplay is much more dynamic and alive. Gen 7 continued to build on this (albeit the wifi plaza is a step back) but I feel it has much more in common with the previous gen than the next.
8 is the first gen with a very new design style, and I'm not talking so much about the region and the new roster Pokemon than I am the game design. It's the first gen with DLC, the first where whole species of Pokemon were excised in the name of gameplay, the first main series game where you can rotate Pokemon in and out of your team without needing to visit a Pokemon Centre. GF's design priorities have visibly shifted with this one, and again it makes for a very different gameplay experience than previous titles in the series.
You do raise some very good points, and I can see where you're coming from. In that sense, I would say that's also why the lineation of eras within the Pokémon series is not totally rigid, barring Gens 1 and 2, and in many ways, the eras I listed still bleed into each other in a sense with the exception of the first two gens which are rigidly distinct from the rest.
I agree that Gen 5 still takes after Gen 3 and 4 in some ways, which is why it's sort of a bridge between 4 and 6 in terms of the shift in the design philosophy. But I disagree that Gen 6 is nothing like Gen 5. While Gen 5 still takes after its predecessors in some ways, many of the things Gen 6 had were things that started in Gen 5, and Gen 5 is where you can see the gradual shift in design philosophy. You say that Gen 6 is where the difficulty lowers, but no, that's Gen 5 where it starts. The whole "Gym Leaders only use three Pokémon max" started in Gen 5, and BW1's Gym Leaders are all really easy with the exception of Lenora which is a matter of whether you know what you're doing or not. The O-Powers that you get in XY and ORAS started as a concept in Gen 5 with the Entralink Pass Powers. The real reason for the perceived drop in difficulty is that the game hands you many ways to trivialize the game and also makes it much easier to level your mons. While Gen 5 didn't have the EXP All, it offers the Audino trick and a free Lucky Egg which makes leveling significantly more quick than in games of the past. XY and ORAS let your team reach obscenely high power levels through Mega Evolutions which all have very high stats, but Gen 5 also does something similar: you not only have Gems as DrPumpkinz mentioned above, but Gen 5 introduced many mons who are effectively broken disc-one nukes on a playthrough. Conkeldurr, Darmanitan, Archeops, Chandelure, and Haxorus all have obscenely high offensive stats that are well over 140, and should you use them, they will ultimately completely trivialize the game. Even less immediately powerful ones have very powerful gimmicks: Krookodile and Scrafty have Moxie which makes them more and more powerful with each kill, Lilligant can set up with Quiver Dance and sweep with Petal Dance, and stuff like Ferrothorn, Galvantula, and Eelektross are also very strong and can trivialize a playthrough through their unique strategies. Gen 5 was the first time your team could reach an extremely high power level that you couldn't reach in past gens.
And while the approach with the Maison instead of a forced 7 streak is different, the structure of the Battle Facilities in Unova, Kalos, and ORAS is alike. They dropped the Frontier starting in Gen 5, and what we end up with in all of these cases is two Battle Facilities: the Battle Institute (which is unique to Gens 5 and 6) and a singular facility dedicated to different battle formats (Subway in 5, Maison in 6). And as I said, Unova shifts towards a region that is completely linear in structure, something Hoenn and Sinnoh weren't. Gens 5 and 6 even hit similar story beats at times, and also have the concept of mandatory capture of the cover legend which wasn't a thing in past games, and the beginning of the use of western regions instead of Japan being used as the region basis.
It's true that Gen 6 did many things differently from Gen 5 as well, but I can also say the same about Gens 4 and 2 in relation to Gens 3 and 1. Gen 4 introduced the physical/special split and a ton of new moves that made many mons significantly more viable, it had features like legendaries respawning if you defeat them after battling the E4 again in Platinum and HGSS, and a heavy focus on wireless interactivity and online events/Mystery Gift. Gen 2 introduced genders, the breeding mechanics, Baby Pokémon, and two new types in Dark and Steel which changed things a lot, plus several battle mechanic changes,
The release structure of Gen 5 is also closer to Gen 6 than its predecessors. While Gens 3 and 4 had paired games whose stories converge in the solitary third version, as well as a remake, Gen 5 forewent an enhanced version in lieu of a paired sequel release. Neither Black and White nor X and Y received an enhanced edition, instead opting for something different that is more of a follow-up and a different experience. Gen 5 got a sequel, while Gen 6 got Hoenn remakes that are also inherently attached to the original paired release. ORAS is notably more attached to XY than any previous remake was, and is effectively an extension of X and Y in terms of story. Masuda even mentioned in an interview
here that both gens forewent the expected Gray and Z for surprises like BW2 and ORAS, which in the grand scheme of things, serve a similar function relative to the original paired games, and under a similar philosophy, despite being very different as well. There's also the topic of how long each generation was: Gens 3 and 4 lasted four years each, while Gen 5 and Gen 6 each only lasted three years. Likewise, the BW and XY animes only had ~140 episodes as opposed to the AG and DP sagas having around 190.
Likewise, in regards to Gen 7, while it's very similar to its predecessors Gen 5 and 6 (especially in terms of its focus on story, let's be real), it's also a bridging generation that leads into the design philosophies Gen 8 ultimately went full force on. Gen 8 was the first that fully did a dex cut, but if you look at it in hindsight Gen 7 was beginning to shift towards said cut: the National Dex was gone starting from Sun and Moon, but every mon was still transferrable so it wasn't a total shift. That said, any mon that wasn't in the Alola Dex straight up did not have a dex entry. While it's the first game that has DLC, the DLC's function in the grand scheme of things is identical to that of a third version. Tying into the release structure philosophy, it's going for a similar approach in that unlike Emerald and Platinum which were both a singular enhanced version, Alola and Galar games opt for a paired enhanced release, especially when you consider that SwSh's DLC got released as a physical bundle at the end of 2020, effectively making the SwSh+DLC identical in niche and function to USUM in Gen 7, both being paired "enhanced" editions of the original pair. And the Box Link concept that Sword and Shield went for is actually a thing that started in Let's Go Pikachu and Eevee, which is also a Gen 7 title.
Also in both the Alola and Galar games you notably start off with much more cash than before. Not to mention Alola was the introduction of regional forms, which Galar did even more of.
In terms of release structure, Gens 7 and 8 are also similar to each other. The paired game, followed up by the pair together getting enhanced (SM->USUM, and likewise SwSh->SwSh+DLC), followed by a revisiting of an old region that does its own thing. Let's Go obviously does its own thing. And while we haven't seen much of BDSP and Legends yet, they are also clearly doing their own thing with BDSP being a more faithful to the roots remake and Legends being a totally new adventure with new concepts that are different from traditional mainline releases thus far.
In fact, between every two gens the release structure tends to parallel, which is something I've been hinting at throughout this post:
Gen 1/2: Base pair->Third version
Gen 3/4: Base pair+Singular enhanced version+Remake
Gen 5/6: Base pair+Paired sequel/attached remake
Gen 7/8: Base pair+Paired enhanced release+Remake/revisit that does its own thing
While this doesn't totally tie into the games, the anime aligns with the gradual shifts in the games' design philosophy over the gens as well. Gens 1 and 2 were their own series with Ash/Misty/Brock and Ash being the sole focus. AG and DP dropped Misty and instead had a new girl companion whom Ash shared focus with, both on their own goals who shared the screentime with Ash. BW and XY also started a bit of a shift in philosophy and tried a lot of new spins despite sticking to the older formula. And then with SM and Journeys they both totally take the anime in a new direction.
In a sense, I suspect that Gen 9 will still be similar to Gen 8 in many aspects, but I also believe Gen 9 when it happens will shift towards another new direction leading to yet another era of Pokémon with a new set of design philosophies that will be brought in full force in Gen 10.
But you still have some valid points in a sense, which is why I said the eras are not totally rigid: rather, the shifts in design philosophy tend to be gradual, with each odd-numbered gen being the beginning of said shifts that the following generation reinforces, even if even-numbered generations also bring about their own chances to differentiate them. Gen 1 and 2 are arguably the most rigidly different era, but from Gen 3 onwards it's inherently subjective and the perception of "eras" can overlap a lot due to how much even different generations can bleed into each other.
