There’s also the aspect that all the filmmakers could be giving somewhat political answers. Muschietti was hired to make a DCU Batman movie, and presumably really wants to make a Batman movie. So he clearly has a vested interest in Reeves’ Batman staying separate.
I also just think that the more Reeves builds out his Batverse, the more his Batman will feel like the “main” Batman and the bigger a lift it’s going to be to get audience buy-in on a second Batman. The optics of two Batmen- ”Yeah, so that other critically acclaimed Batman over there is too good for our wacky superhero universe.” The optics of merging is them is “We are a strong and diverse brand that can embrace multiple strong creative visions under one umbrella.”
Now as far as how it would work out if they actually did it, I have no idea. I will say, tonally and aesthetically, merging a darker take on Batman with Superman and the wider DCU in a movie is still always going to be a very tricky task. I still don't think it's been done totally successfully. But it’s still an interesting idea to ponder.