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.
Cochrane and Sloan probably couldn't have helped getting somewhat inspired by what they saw of the Enterprise-E
and yet when it returns to its future the technological level is the same.
I maintain that the drama in The Voyage Home
or "Endgame", and the universe in general, comes from the idea that there's only one timeline and set of characters that can be changed, otherwise there's no point in trying to change, or prevent changes to, the past.