I've seen repeated in several places that some people don't accept the alternate timeline aspect because, as has been said, it's been shown time and time again that time travel doesn't create alternate times. First Contact, Voyage Home, City On The Edge Of Forever, etc. To which I say.....how do you know?
People just assume that FC didn't create an alternate timeline because Picard & crew went back and confronted the Borg. But why? If we're accepting the Trek 09 theory, there are infinite timelines. There is no original. There's no reason to think that Picard & crew just went back to exactly where they were. The one before the Borg went back and the one after the Borg were stopped, just happen to be so similar to one another that no discernable damage was done.
There's another timeline where they weren't
stopped. The Ent crew sees it on their way to back in time.
"6 billion lifeforms. All Borg."
To me, the "Prime" Trek universe is the one with the least damage was done. After Kirk & crew returned from the 1940's with McCoy. After they returned from 1986. After First Contact.
I'm partial to the timeline where Star Trek is a franchise and I'm just a guy talking about it.