Yeah I don't think that by itself it is a deal breaker for -that- many people, I'm sure most people are somewhat flexible one way or the other. I -do- think it might play a big part of BG 3 struggling with its legacy and changing identity.
What I mean to say is, the combat system might be a significant part of a whole, that creates the "deal breaker" (or disappointment, or alienation of old fans, or whatever).
Because the game is TB, I think it becomes extra important that Larian separates BG3 from the DOS games in other ways. Here comes the vague terms: atmosphere, visuals, audio, UI, writing/narrative, and other parts of game design outside of combat mechanics.
Because if they don't do that, they could've just named their game -anything- other than BG3. It'll just mean it was a pure marketing move with zero artistic vision, and as BG fan that just sucks.
I hope it being TB will contribute to great combat as part of a great game, but I also hope that it being TB won't contribute to a series of reasons why this game will (hypothetically) fail to live up to its legacy. These 2 "outcomes" are not mutually exclusive.