While I agree with 90% of what you said. A little bit of it depends on things I don't know.

For example, the last Legends game was compatible with Home to allow its mons to transfer to mainline games. Will this be the case again, and if so, will Mega Evos be included in Scarlet and Violet? If so, I wouldn't say that makes them a spin-off. At least no more a spin-off than a dlc would be. It still impacts the main line games in this scenario. Granted in a way that's not relevant to this discussion, but in personally hoping for Megas in Scarlet and Violet.

In any case, i would also argue that the addition of Fairy typing was a huge shake up to battle mechanics, because it's not JUST a turn based combat. There's also equal parts rock, paper, scissors involved. Adding another option that interacts with existing ones is a huge change. Before that they added double battles.

I agree with the spirit of everything you said, but the formula has gotten updated. It's just not the side of it that I guess most people would think of as "the combat"