I keep my theory that this decision of cutting Pokémon now wasn't because they couldn't do it for the Switch, or that they couldn't re-balance whatever problems there are in the battle system, or because they were going to give us something out of this world in replacement. What I mean is had they decided since the beginning that they wanted to include all the Pokémon, they would've done whatever was needed to do so, including planning a later release date (before anime, TCG, merchandise, etc got the memo that the new generation was coming in November 2019) and they would've hired whoever they needed and how many they needed to hire, and cut any other feature that would've taken time or resources from the main goal: to include all the Pokémon.
But for some reason, they (whoever are they, whether is GF, TPC, or a series of entities we don't even know about) decided that they were reaching an unsustainable number of Pokémon, and that this number keeps increasing with every generation, and that it was time to stop. If they asked me, I would've said that they should wait at least one more generation (actually I would've been against cutting Pokémon even if they were 15000) but I understand that the Switch era has just started, we don't know how many games Gen VIII is gonna have, but it's most likely that the Gen IX will come for the Switch too (just like how the DS got two gens, and so the 3DS).
So if they included all the Pokémon in these games, SwSh, they would've been forced to include them not only in all the Gen VIII games that are yet to be released (which wouldn't have been a problem) but also, they would've been forced to include them in the Gen IX games (which are gonna increase that number even more). Tbh, I still don't see the problem lol because it's not like the Switch is limiting them like the 3DS did, and it's most likely that we'll get even more powerful versions of the Switch in the future. I'm not saying it would've been easy, but it was possible to include all the existing Pokémon + a few new ones in these upcoming games. I see it like they tried to break the chains now before it was too late and actually got in the trouble of having to deal with 1000+ creatures. So any excuses about balancing the game or that they're giving us better features in animations or story as a compensation, I just ignore that because, at least to me, it's obvious that's not the real reason.
At least that's what I keep telling myself or otherwise I'll drive myself crazy over-analyzing every trailer and every little thing we see there to ask myself if it was worth the cut, because, at least to me, nothing was worth the cut, but I understand it's a decision they made thinking about the future.