From my understanding, ST'09 was originally rewriting/replacing the timeline in the same physical universe, just like DofP, and the producers only changed their stated intent with the fan backlash.....
I'm not sure if that's the case. Given that at least two members of the "Supreme Court," Roberto Orci and Damon Lindelof, were themselves passionate Trekkies, and Alex Kurtzman a moderate Trekkie (IIRC), there were enough fans involved from the start that I doubt any external backlash was needed to defend the original timeline. Also, Orci's online comments back at the time revolved around his research into quantum theory and what it said about alternate timelines, which is that they would coexist rather than replacing each other. So it came from getting the science right, no backlash required.
Its really open for debate on whether changing the past can honestly create another physical universe that exists alongside the original one, or whether it is all one physical universe that can be rewound and re-recorded on.
That depends on whether you're speaking in terms of fictional plot devices or real physics. The "re-recording" idea may be popular in fiction, but it's scientifically absurd. The idea of "changing" a moment in time is logically contradictory; change requires a "before" and an "after," and a single moment in time cannot come after itself. So you can't "replace" one version of that moment with another. The only way there can be two or more versions of that moment is if, by definition, they coexist simultaneously.
In real physics, there are only two possibilities: One, there's only a single, immutable timeline, and any time travel would be a self-consistent loop creating its own history; and two, parallel timelines coexist and a time traveler would create an alternate timeline alongside the unaltered original. Indeed,
by the quantum-mechanical model formulated by David Deutsch, time travel is probabilistic -- if there's a chance that history will be changed and a chance it won't be, then the time traveler will follow
both paths, themselves splitting into one version that succeeds in changing history and another version that fails. People tend to assume that the closed-loop model contradicts the branching model, that it has to be one or the other, but the Deutsch model says that it would always be both. Which is an idea that doesn't get explored much in fiction, if at all -- the notion that any time traveler seeking to change history would split into two selves, one fated to succeed and one to fail. (Or several fated to succeed or fail in different ways, depending on how many potential outcomes there are.)
So either way, the original history must survive unchanged. If something happens, it happens. It's part of the larger equation of the universe and cannot be erased from that equation. If there are alternative versions of the same events, then they are part of the same overall probabilistic equation of the universe, meaning that all possible outcomes coexist. One cannot "replace" the other. The perception that one timeline is changed into a different one is an illusion caused by the time traveler's worldline crossing from one timeline into the other. It's only in the traveler's subjective perception that one comes before the other. Objectively speaking, they both occur simultaneously.
