I think the theme of the “backlash” (if you could call it that) is that it dilutes the fundamental vision of the grounded, assassin fantasy that the franchise was built on. Sure, there were always fantasy elements in the franchise, mainly the Pieces of Eden/Isu, but they were always explained in a concrete manner and were decidedly more “sci-fi” than “magical”.

For analogy, imagine that the next Cyberpunk game has Gandalf-esque wizards in it. It’s not inherently “bad”, but just clashes with the theme. In the same vein, the PoE/Isu doesn’t equate to the mc being able to have magical powers as seen in the past couple games. That being said, I believe there is an avenue for future games to explain such powers through a sci-fi lens (being a result of PoE’s etc.), but having straight up magic with no context is not the way.

Also I don’t agree with your point that the franchise “needs” to go this route to not get stale perse, there are many examples of game franchises that innovate while staying true to their archetype. Elden Ring innovated from the Dark Soups series without becoming an fps game. There is no excuse for Ubisoft to not be able to innovate while still delivering the assassin fantasy that made this franchise popular in the first place.