>Gen 1 is obviously all new mons, but the mon distribution is fucking whack, with lots of routes being the same, stupid places like safari zone being the only way to catch certain mons, some mons being trade exclusives like Lickitung/Jynx, etc
>Gen 2 is when they didn't know what they were doing, it's oops all Kanto mons again with many of the new additions being obnoxiously rare in Johto areas or are somehow in fucking Kanto
>Gen 3 is a soft reboot and adds the most new Pokemon after Gen V, routes are varied with new mons but also gives you access to old mons, as well as a safari zone where it's only old mons so for majority of people they'll likely be using majority gen 3
>Gen 4 has half of its fucking "new" mons be evolutions to old mons, routes are horribly designed with you getting Machop in Bidoof and other garbage constantly. The only reason every persons team is the same is because those are the only new mons that are simple to get
>Gen 5 is all new mons and the most added in a new gen to date so you're forced to use all new mons which is very good
>BW2 since it's not a third version and instead a sequel, where the devs likely expected you to have already played the first games, now have access to older mons in the main game but still has all the variety of the new mons at your disposal, so whether you're a first time player or someone who has played every game you can use any variety of mons

I don't know about it being necessary or justified or whatever but I consider it a good decision. If they kept it the same as BW then every single route would likely have ended the same. The variety is what kept it fresh

Anyways rate my team for my recent White playthrough