I'd say it's debatable considering that D/P/Pt had Dawn and Lucas battle Dialga/Palkia/Giratina before the 8th gym depending on which game you had, I'd have to consider that canon (the only difference is that you could kill them). The only difference between the two is the mythology attached to it, we were forced to catch Reshiram/Zekrom in BW1 because the Player Characters were set up to fight each other and continue the Unova brothers conflict (think Uchiha vs. Senju > Sasuke vs. Naruto if by any chance you follow Naruto). Dialga/Palkia was a result of Team Galactic's ambitious plan, though you can plausibly argue both sides, defeating them keeps Sinnoh safe, and catching them also keeps Sinnoh safe (it's low leveled enough to argue that it can be caught and kept under player's control).
Undoing their mistakes is a redundant storyline, considering they're already defeated and have no galvanized motivation to start some shenanigans again...not to mention it makes the previous journey such a waste of time.
The point of 1st gen Pokemon is the fact that Johto and Kanto were an interdependent environment, so it shouldn't really come as a surprise that there were so many 1st gen Pokemon in Johto. Plus in its time, it was to be a canonical sequel to the popular RBY, so in that regard it's also understandable why there were many Kanto Pokemon available. I don't think we need to be bombarded with every Johto Pokemon for the gym leaders, I mean Pidgeotto, Scyther, Gengar are still good and/or annoying the same way Kingdra, Miltank, and Steelix. Plus 50% of Ace Pokemon devoted to each region makes for a nice balance, I think. I don't think Hoot-Hoot, Spinarak and Ledyba would've made much of a difference considering they're horrible. I think rehauling the Johto dex to include 5th/6th gen is being a bit too optimistic considering we're not even sure if we're going to revisit Johto or any of the old regions anytime soon.















