Let's start with a few assumptions:
- Within any given timeline/universe (which we shall treat as a closed system), there can only be one Booker/Comstock at a time (minus external interference from tears).
- All timelines/universes without a Comstock in them don't matter (obviously).
- All timelines where Booker was removed via a tear don't matter (see above).
- All timelines where Anna DeWitt remained Anna DeWitt, or was never born, don't matter.
Since infinity minus anything other than infinity is still infinity, the above doesn't mean a whole lot quantitatively in terms of the total timeline/universe tally, but it does introduce a concept that isn't readily apparent to us... that the number of timelines/universes that exhibit the above may in fact be finite. Within infinity, there can be finite subsets. In other words, not everything within a infinite set is infinite itself. Tilpo, feel free to step in with that cardinality, axiom of choice, and axiom of determinancy stuff if you like.
So okay, now we can get somewhere. In addition to the above, let's also consider that in order for the storyline to play out, there must be an interaction of two timelines/universes. What I mean to say is that - given the assumtions above - the only timelines/universes that matter are the ones where Booker was not removed from the timeline/universe, became Comstock at some point, then kidnapped Anna DeWitt from another timeline/universe at some point, AND had a Booker DeWitt pulled in from another timeline/universe at some point (presumably from the same timeline/universe that the aforementioned Anna DeWitt was kidnapped from). This scenario involves at least two timelines/universes. Moreover, the timeline from which Booker was removed immediately becomes irrelevant as there can be no Comstock there.
I know I'm probably not being too clear about this, but what I'm trying to say is that each instance of this storyline is in fact a member of a self-reducing finite set. And that means, that given enough instances, it would in fact be possible to wipe out all such members over time. So what does this all mean? Simple, it means Ken Levine ****ed up and should have just named the game Bioshock Columbia.
Or alternatively, that damned hummingbird is the evil mastermind behind all of this and we just haven't figured out what his/her endgame is.