An actual technical analysis of the whole reused model situation.

Notes:

tl;dr:

-There's proof some pokemon use alternate models during dynamaxing, like Pikachu using Let's Go's starter Pikachu model (which has extra details on the mouth).
-While Ohmori's argument about redoing models from scratch is for the most part pure balloney, it does hold some grain of truth. The amount conversion, tweaking and time required for each one – due to GF swapping graphical engines often – isn't easy peasy as one would believe.
-It's highly likely all of the pokemon models would've made the jump to Sword & Shield hadn't development deadlines been a bigger priority than porting all the creatures to the new generation.

Overall, I'll say this one is pretty informative and it's a must read for all interested parties.