I like Watching the Clock
's interpretation of things - that it was the method of time travel in Star Trek
that caused the alternate reality to split off. The black hole was a strictly one-way transfer, so the resulting timelines weren't entangled and so don't merge as the ones using two-way time travel ("Yesterday's Enterprise" etc) do. That way classic Trek's myriad time travel stories (and future ones in the novels) aren't rendered obsolete.