If your thought was that they'd slowly bring every Pokemon back up to some new animation standard, the problem is just going to present itself again every single time they want to add a new feature. At absolute best, you'd be dealing with a dexit every gen they would be undone by the very end, and it'd just get harder and harder to do as the dex continues to expand.
Even without considering the competitive game, I'm pretty sure a complete dex in a main game is gone for good. Although - you have to wonder why they can't all be in BDSP. There's not really anything going on there that wasn't in USUM.
The idea is we're already past the largest hurdle in Pokémon of the last decade and that's the full jump to 3D environments, so yeah while more marginal upgrades are possible I'd rather see everything standardized again in the SwSh/Legends art style and maintained for the next decade until the next major creative step.
Imagine they'd have to demake Gen 8 models into Gen 6 sprites for BDSP and subsequent remakes though 😅