It needs to be balanced, no question. But you can't go cold turkey. They tried that in Unova, and that backfired badly, as b+w were not well received. Maybe in the main game, have less older Pokémon, but in the current get up, the expansions each generation should absolutely include older Pokémon. I'd love to see more regional variants. Take it a step further: since we have time travel and apparently a multiverse, remake a few gens with all regional variants.