So if Mulgrew had dropped dead who would they have made captain? I have trouble believing it would have been Chakotay as the guy is really too terrible an actor to have a show focused on him. Would have been interesting if they had him as a background captain like Weir in Stargate Atlantis ended up being (presumably because she was an even worse actor) and made all the stories about the rest of the crew but that seems a bit radical for Star Trek. Would have been interesting though.
I bet they would have brought someone else in entirely, found some stranded starfleet ship with a hot admiral on it.