The 2009 film SHOULD have been a straight-up reboot. After all, how many versions of Batman have there been? Of Superman? Sherlock Holmes? Dick Tracy? The Twilight Zone? James Bond (okay, THAT one's a special case because despite the change in actors, it's intended to be the same series. The inconsistency of Judi Dench's M being new to Brosnan while at the same time also being there when Craig's Bond first joined the serivce notwithstanding)? Battlestar Galactica (although, I HATE to even acknoweldge Ron Moore's version)? It's not like we fans wouldn't have been receptive to that. it's not a hard concept to understand.
But that's just it -- Trek fans have spent many, many years screaming and arguing on the Internet about every niggling continuity glitch and inconsistency, while other fans have said many times that they have no interest in reading Trek novels or comics that don't "count" as canon. Trek fans -- maybe not the majority, but the vocal minority who dominate the public dialogue -- have a history of being unusually hostile to the idea of alternate continuities, an aversion that they seem to share with few other fandoms, notably
Star Wars fandom. So the filmmakers were convinced that they
had to connect the new continuity to the old or Trek fans would never accept it.
Of course, what they didn't realize is that the purists have been rejecting
every new interpretation of Trek as "wrong" since the animated series first came out 40 years ago. As we've seen, their attempts to tie the new reality to the old haven't won over the purists anyway. So I agree, ideally they should've just made a completely fresh start. But they had good reason to believe fandom would be hostile to that, because Trek fandom has spent decades being vocally and aggressively hostile to alternative continuities.
Which is partly because we have no experience with them. Unlike many long-running franchises, all the various revivals and sequels and movies and spinoffs from the original have all presented themselves as being continuations of the same reality rather than alternate versions. TNG, as I've mentioned before, may have been intended by Roddenberry as a soft reboot, selective about what it acknowledged from TOS and what it overwrote, but later producers tied it and its spinoffs more directly to the original. So unlike, say, Batman or Godzilla fans, we just haven't had the opportunity to see a completely separate Trek continuity, and thus the idea is unfamiliar.
Nothing's getting erased, don't worry. The novels already spelled out the black hole/AU concept in the first DTI novel.
In fact, in
Watching the Clock I offered
two physical explanations for why the timelines would continue to coexist. One was the idea that a one-way time travel wouldn't mutually entangle the timelines enough to bring about quantum convergence. The other was that if the geometry of spacetime were sufficiently altered by, say, the destruction of a planet, then the two timelines would no longer align in space sufficiently to allow their particles to converge. In effect, one would be "bent" out of shape so that the two timelines could no longer "fit together" well enough to merge. Which sounds silly when I put it that way, but there's solid physics behind it.
