From someone who's never actually done the startup thing: when you feel like your product is good enough to stand alone, outside of the specific domain you originally targeted for, you likely have to have a second round of "initial" user acquisition, where you cater to, and iterate on, the needs of more general users you recruit.
But if you try too hard to get that one domain, you may not reach a point where it's "good enough to stand alone, outside..." because, while your execution was good, you moved in the wrong direction.
