Okay so it's time to talk about the EXP Share and overall talk about level curves in general. I've more or less spent the past several months playing most of the Pokemon games and marathoning them, but here's what I think about the EXP Share mechanic, and how it's related to overall difficulty of the games.
But the main unpopular opinion I have here is that the EXP Share isn't making the games easier. What it really does is that it makes the games less tedious. EXP Share by itself makes spreading the EXP across the party much easier so now everyone gradually levels up in strength more quickly. And as a result, it's easier to keep up with the level curve of the game and keep your whole team up to par with the levels of the Trainers you face across the game. Especially if you're using a full team of six. In that sense, it reduces the need for extensive grinding and allows you to not need to put in the tedium to make sure your team is up to par with where the game wants you to be level-wise at each point.
The Pokemon games in general are not that hard. Build a good team, understand type matchups, and be on part or just a little underleveled with your opponents, and you're all set: you can just beat them easy peasy because none of them have EVs or competitive movesets. Sun and Moon were so far one of the few examples of a legitimately challenging game because the Totem Pokemon had genuinely good movesets and teammates to create truly compelling battles.
Gen 3 and Gen 4's level curves created artificial difficulty by being steep to the point where you need to grind extensively to catch up at various points if you want to keep up with the level curve. And there was a way to do that: they had Trainer rematches. The amount of experience you can gain without Trainer Eyes/Match Call/Vs. Seeker in those games is pretty thin, so if you use a full team of six, you're gonna be underleveled without relying on the Trainer rematches. As an example, look no further than Lueroi's very well known Sapphire and LeafGreen walkthroughs, where his teams were clearly horribly underleveled. So you need to grind, especially in RSE, FRLG, and DP and HGSS, and basically the game wants you to backtrack on a regular basis and find all those rematch-upgraded Trainers and rebattle them at every conceivable point. If you do that, your Pokemon will be fine in terms of levels and will be up to snuff with the level curve, and the games aren't too hard if you do that to catch up. But that's not actually challenge. That's tedium. That's putting in extra time and effort to raise your team so that they catch up, including the extra backtracking you have to do throughout the region to go find all those rematchable Trainers all over again and battle them.
As an example of how much time this adds, in my latest Diamond playthrough where I constantly backtracked and found every possible Trainer at every opportunity and rematched, including Trainers in hidden places via Surf/Rock Climb, once I was at the League my playthrough was around 30 hours long. After the post game I had spent around 50 hours. Meanwhile, in my Y playthrough, after doing practically everything including the post-game, I had spent around 25 hours. That's half the time I spent on Diamond.
My BW1 playthrough after doing everything up to Alder was also around 25 hours long. But in this case, the games may not be that hard, but the key thing is that Gen 5, despite not having the EXP Share, had a genuinely good level curve and combined with the EXP system of that generation, made sure that you were more or less up to snuff with a good team level wise. BW2 arguably was even better and had a very smooth level curve including the post game (whereas BW1's post game levels spike immediately from the low 50's to 62-65). There were also very easy training spots.
Which leads to the other problem, which is that Game Freak clearly optimized XY and SM's level curves in such a way that the EXP Share should be toggled. In other words, constantly turn it on and off at various points. XY's level curve is very strange in that it flatlines at certain points then becomes incredibly steep at other points. There are quite a few notable level jumps in both XY and SM where you absolutely need to use EXP Share to catch up. I found myself actually toggling the EXP Share on and off at various points, as from my experience with XY, if you keep EXP Share off the entire time, you will end up sorely underleveled for the League. Keep in on the entire game, however, and you will be severely overleveled and curbstomp everything in sight. There are clear points in the game where you are expected to turn it on, namely around the Reflection Cave point, and then are expected to turn it back off at other points. Same with SM, where it's ideal to keep it off for most of Ula'Ula and Akala but turn it on towards the end of Akala and the whole Aether and Poni segments as around Poni the level curve gets really steep.
There's a problem there where for the most optimal experience, you basically have to turn EXP Share on at different points to catch up to a steep level jump. But that requires you to keep track of whether you have EXP Share on or off and keep up with the curve. Granted, however, this still does reduce tedium, as while Gen 3 and Gen 4 run off a similar level curve style as Gens 6 and 7, the older gens made you use Trainer rematches to grind your team which adds tedium, while the Gens 6 and 7 had a togglable EXP Share mechanic, which while less tedious, requires you to go into the bag and turn it on and off several times.
That being said, the use of EXP Share in newer gens does in fact, reduce tedium. As I said, it's not the challenge that's going down in recent games, it's the amount of time and effort you have to spend on training your team that's getting shorter. But even then, however, Game Freak's level curves still make it so that EXP Share should be on only at certain points, so it's still not perfect in that regard. Pokemon isn't, and never really has been, a challenging experience. The older games were just a lot slower, and necessitated significantly more grinding and time to keep a team of 6 on par with the level curve of those games. In other words, a lot grindier. So in other words, the newer games are about as difficult, per se, in terms of the actual challenge of the battles, as the previous games. But they require less actual grinding to keep up in terms of experience, and the EXP Share ensures that that tedium is reduced. Which is why playing the newer games feels "easier", per se, it's that the games require less time to catch up with the level curve and thus can be completed efficiently more quickly. Thankfully, Sword and Shield, despite having permanent EXP Share, had a pretty well designed level curve overall, albeit still a little flawed on that front and the gyms were still on the easy end (Leon on the other hand though is legit a great Champion and had a really good team).
Now granted, there is perhaps a charm to that old tedium that people find likable and why they would prefer having the EXP Share off. Investing that sheer level of time and effort to raise a team by backtracking and grinding, and putting in the effort to raise each and every one of your Pokemon, can feel incredibly rewarding. So while the older games are still easy, per se, they were more grind-heavy, but because of there was a lot of grinding, raising your team to such a high level by the end of the game feels rewarding and extremely satisfying as a result: all of your Pokemon are where they are now because you worked so hard with each and every one of them. Even though the EXP Share reduces tedium and grinding, there's a psychological downside to that: it means that getting your team to become high level and strong doesn't feel as rewarding anymore because you basically just did the bare minimum to make your team strong. Like, wait, that's it? So while tedium is pretty old school, older fans probably liked that because the grinding through Trainer rematches can still have that psychological satisfaction when you do finally beat the Champion and the League and complete your adventure.
Now perhaps, a best of both worlds would likely be ideal. BW2, and Gen 5 overall, had the best designed level curve of all the games and ensured a very smooth experience without an extensive need for grinding. And that was where the current EXP Share mechanics didn't exist. Gen 6 and Gen 7 had the EXP Share as an optional thing, and perhaps that's for the best maybe. Or they could make that EXP Share permanent but better optimize the level curve.
But the main thing is, as far as actual challenge and difficulty goes, the level curve alone isn't what will constitute genuine challenge. Good teams, good Pokemon with good movesets, and IVs or maybe even a bit of EVs will go a long way in creating a genuinely challenging Pokemon experience. The Totem Pokemon, while taking it a step too far, are a pretty good example of what real challenge is. But granted, challenge should likely be an option and going back to Gen 5, having a Challenge mode like BW2 did, except now with Pokemon that had legitimately good movesets, items, EVs maybe, and vice versa, will go a long way in creating a good experience for veteran players. After all, Pokemon is a franchise meant to be kid-friendly, and so the current overall difficulty of the games is fine as a default, but there should be options as well for the more veteran/established people who would prefer something challenging. BW2 at least constitutes the best designed examples of a well designed level curve and having good challenge.
I'm rambling at this point, but this is basically my main point about EXP Share: it alone does not make the games easier as much as it does reduce tedium. It reduces the need for grinding that older games absolutely mandated to keep up with the level curve, but even then Game Freak hasn't totally nailed making a level curve that works perfectly with it always on, which is also another issue that needs to be approached. That being said, the current EXP Share in and of itself is a fine concept and it's not an issue when it comes to the issue of difficulty of the games.
(note: i've never been to the UK, any comments made here are in jest).
