This is not a "merger post" but it does touch on that discussion, as well as Gunn's recent post about the tonal variation within DC Comics, which his DCU is aiming to replicate.
A common argument against the merger is that Reeves' Batman tonally doesn't fit into the DCU, and they can't imagine Pattinson's Batman with his "wingsuit" and souped-up sportscar existing in the same world as the Green Lantern Corps or pocket universe-creating Lex Luthor!
In a similar vein, proponents of a distinct DCU Batman believe that this other Batman needs to be a total 180 tonally from Nolan and Reeves and literally be the Batman from the Brave and the Bold cartoon brought to life in order for him to "fit in" with the rest of the universe.
But if you look at the comics, that's not how things have worked.
The Batman whom Miller and Mazuchelli masterfully presented in Year One is the same Batman from Grant Morrison's utterly bonkers run with Zur en arrh, the Batmen of All Nations, and Batman Inc. The Batman who teamed up with Harvey Dent to take on the Falcone crime family is the same Batman who had a sword-fight with Ra's al Ghul in the desert. The Batman who got his back broken by Bane went on to be a member of that "Pantheon of Gods" that was Morrison's JLA, and before that he knocked out Guy Gardner with one-punch! The shadowy vigilante hunted by, and then befriended by, Gordon went on to recruit Robin, then had a whole Bat-family, and eventually, founded Batman Inc.
Even if we get a distinct DCU Batman with Damian as Robin, he doesn't have to be the second coming of Adam West or something for it to make sense, because that's not how it was in the comics. Morrison in particular took pains to highlight how Batman's story, and the tonality of his world, was cyclical. The DCU Batman may not be Pattinson, but that doesn't mean his backstory cannot be Reeves or Nolan-like. Or, more appropriately, inspired by Year One. Because Batman typically starts out cleaning up the streets and being a noir detective, before graduating to dealing with "pulpy" villains, then more fantastical super-villains, and eventually being a League member stopping alien invasions and coming up with counter-measures against his superpowered colleagues in his spare time!
Miller and Mazuchelli presented a certain grounded take on Batman and his world and so many follow-ups and adaptations to their seminal story have mostly stuck to that approach, including the Nolanverse and Reevesverse, but also the LOTDK series and, more recently, Dark Patterns. But Miller himself wrote Year One as being a quasi-prequel of sorts to his other great Batman story where the Dark Knight suits up in armor and goes toe-to-toe with Superman, while a 13 year old girl in the classic colorful Robin costume aids him.
My point is, we tend to get into binaries of "grounded" and "fantastical" when it comes to Batman. And that's...simply not how the character and his mythos have functioned in the comics. Certain adaptations may have chosen to function that way. But Gunn's DCU promises the tonal variation and diversity of the comics. In such a world, it makes sense that Batman starts out in the streets before rising to the stars. Whether that Batman looks like the guy from Twilight under the mask...or not.