I have had no issues using pokemon from after Morty at all. Hell, the only pokemon I would say actually suck beyond use is Wobb and Tyrogue and not 100% sure I would even say that about Ty. Sure you can't use a bunch of mons from that far back, but that just adds some fun team planing to the game. Steel and Dark types being uncommon is because the types were new in the original games and there wasn't a ton of mons to retcon. There is plenty in After Game, gyms (8 of them all just as hard as the rest of them. Which is easy but still) Basically a super end game boss in Red. Even if the frontier is copy pasted from PL, it is still there and is end game material to do that quite a few of the games lack in. You get little side quests like repairing the train. As far as your argument that fluff doesn't effect gameplay, it totally does. It gets you invested into the world. This is a good thing, because it makes you want to explore, which you know, helps you find the odds placed mons, which in turn can lead you to experiment with them. It also makes the experience more memorable. Fluff keeps you playing, which in turn makes the game better. This is why I am sooo happy about all the fluff stuff they have been showing off for S/M. It might actually make me what to explore something I haven't cared to do in pokemon since FR/LG. The old characters also showing the advancing world, and putting old favorite pokemon in the game make people care more and draw them in. (Which is good game design from the sales point of view.) I guess what I am trying to say is what makes a pokemon game good and what game designs are good is SUBJECTIVE. What someone likes design was will vary. For example you like the 5th gen exp system. I hated it because it made grinding to 100 a pain. So for you it was a good game design, for me it is terrible. (Actually it was the worse thing they did that gen to me). So, stop trying to act like your opinion is the god dam fact.
Fact , Pokémon of lower level gain more experience in BW, this games allow you to experiment with recently captured Pokémon more than any other game up till XY exp share. I do admit it is disheartening to see less exp on the league after every run, ingame it was a good thing , post game the penslization was poorly implemented.
Back to Johto and my worries with fluff as a focus on games.
Fact , Johto is a game that was in development hell until Iwata compressed the data and optimized two regions into what was supposed to be the space of two.
Fact, Fluff doesn't change gameplay at all, point and case DOOM and its clones as the best example in gaming history.
Fact, the obtain rates of several johto species can go as low as 1% in the case of Marril.
Fact almost all of the johto families except the cases I listed previously have terrible level up movepools that require too much sustain on a game with a terrible experience curve unless grinding happens.
Fact almost all the johto lines are 1+ offensively by the Kanto lines both in movepool and bst , johto lines tend to be better for the support role sans 3 exceptions in the original and 7 in the remakes thanks to updated movepools/crossgen evolutions.
I'm not pulling this out of my ass, stuff like Yanma suffer a lot in this game unless you grind psychic in voltorb flip or get lucky in goldenrod, stuff like Heracross got blessed by brick break in the remakes and Machoke became a God with the addition of fire blast and focus blast coupled with no guard until Kanto where the level curve actually gives it a move to outdamage focus blast from its superior atk stat after lv 37, I do a lot of conscious runs through my games and the fact is that johto harbors several issues on a technical level that can make the game experience of using certain Pokémon more mediocre than others, I dare you to do a run in less than 20 hours without a game corner Xatu, you will be spamming wish+fly until Kanto gives you psychic or you learn it mid 50s, so much for a backtracking reward when Hypno, Slowbro, Kadabra and Espeon cover the role better, heck Kadabra, Hypno and Slowbro outperform Espeon in GSC thanks to the elemental punches.
Enjoyment is subjective and part of multiple factors, the fluff of pleasant visuals and audio is nice to have, but core elements being missing and a true sense of rewards for party experimentation is REALISTICALLY missing from GSC and HGSS as Pokémon games, that is not subjective is a fact due to design flaws. People can't try to defend Johto with their subjective views but I won't reply further , we see clogging the thread and I already made my comparison on mixed dexes , exp curve and design I can't link my analysis of Johto's faults without repeating myself on the little information we have of Alola without sounding like a fatalistic guy and I'm honestly trying to stay positive here.
On charjabug...I wanted to use this in triples with tapu koko and Alolan Raichu, RIP triples.