Honestly, the new Pokémon. I can live with any Gen 1 Pandering or desperate attempts to get the mobile crowd on board. I'll grumble about it, but I'll soldier on.
I moan about a lot of stuff with the 3DS games, sure. But when a new generation starts, I could always rely on a bunch of new Pokémon to discover. And then I could experiment with them online to see what new strategies I can come up with.
The competitive scene changes quite a bit when you introduce new Pokémon with new moves and abilities. A new popular threat can suddenly make a previously underutilised Pokémon suddenly carve itself a niche. (Marowak, a Pokémon that hasn't been good since Gen 2, overnight became the best counter to Pheromosa. And quickly became the defensive core of my team throughout Gen 7)
When I first played Pokemon, my sense of discovery was satiated simply by watching my team throughout the story. 9 year old me ran to my mum, getting over excited about how I finally got the cool dragon on the box.
Nowadays, finishing the Pokédex marks the beginning of my experimenting. Finding out how to utilise the new Pokémon I really liked during my playthrough, what other people are copy pasting from Smogon and figuring out new and unexpected ways to punish them for having no imagination. (No Kyogre can stand against my Speedy Thunder Punching Groudon)
Say these next games went the "Return to Kanto, only Gen 1 Pokémon" route. Going by the official rules of "Only Pokémon that can be caught in the latest games can be used in competitions" That's effectively reduced the team building options by more than half! And there isn't even new Pokémon to help mitigate that. And that's before we get to the issue of how most Gen 1 Pokémon aren't even viable...
I don't believe Game Freak would go so far as to having only 151 Pokémon. At worst, they'll probably have some modern Pokémon available after you see the credits. But without 70-ish new Pokémon to play with, I might as well just carry on battling on Sun and Moon.