If it was set in Future State's universe it wouldn't be in main continuity. And the books are absolutely leading to Jace becoming Batman. Scott Snyder has already confirmed that it'll happen, just not in the same manner that it did in Dark Detective and Future State: The Next Batman: https://aiptcomics.com/2021/01/05/sc...xit-interview/
And John Ridley has already stated Jace will be an ongoing Batman, that the next Batman will be connected to the rest of the Bat books, yada yada yada.
And on this point, I think it's some of Bruce's fans that have the perception that he's being replaced. As Joshua Williamson confirmed on the wordballoon interview yesterday, no one's replacing Clark or Bruce. In Jace's case, it's more of a shared mantle akin to Miles Morales being Spider-Man.
I'm Batman
Personally, either one is fine with me...