"I’m going to try to explain why it could be that Booker’s death does kill every iteration of Booker/Comstock, but this comment is probably going to have to be kind of long to do it.

The key to understand is that not every universe you and I can imagine is possible (and at the same time not every universe that is incomprehensible to us is impossible). This is because human brains did not evolve to simulate and understand the universe at its fundamental basics, but instead on the level that we interact with it. We interact in a world filled with things like air, dirt, rocks, people, etc, but in a sense those are all just useful illusions we use. In reality all of them are made up of various configurations of quarks and leptons. However, understanding reality on that level takes A LOT of computing power. Even if one of our early ancestors might have understood reality at that level, the amount of time they would have spent trying to calculate out quadrillions of quark/lepton arrangements to figure out the world around them would have assuredly meant they would have been tiger food or starved to death trying to tell the food quark combinations apart from the not food quark combinations. So we have a bunch of surface rules and understandings of the universe which allow us to survive long enough to reproduce. Our imaginations run on the rules which have let our ancestors reproduce up to this point, not the rules which reality actually runs on.

To examine this, let’s think about what old Elizabeth told Booker about beating Songbird. She has seen all the possible worlds, and in no possible world can Booker defeat Songbird and take Elizabeth away to live happily ever after in Paris. This is not invoking the rules our imagination works under. We can imagine Booker getting a big enough gun, a lucky enough shot, or just destroying any Songbird calling machine on an airship he takes Elizabeth away in. But those universes are not possible in the deepest rules of this universe. Anytime Booker tried one of those strategies it would fail for some reason or another because otherwise the universe enters into a contradiction on it’s most fundamental levels and ceases to exist.

Which is exactly the issue that leads to Booker always in all worlds drowning in that river. In every world in which he somehow avoids drowning he creates a contradiction in the most basic laws of the universe, which will force him to return and kill himself properly.

At the level of human understanding we happen to be calling that contradiction Elizabeth. She is the universal equivalent of a divide by zero error.

What we play through in Bioshock Infinite is actually not a possible world in the deepest possible sense. Elizabeth can do impossible things because in fact she IS impossible to begin with. She is the correction that ensures that every universe fails to make the departure from what is actually possible as represented by Booker’s failure to die. Every world she exists in will lead her to getting Booker back to that point until he makes the right decision.

Of course, this description is at the level of human understanding and what is really happening is an arrangement of quarks self-destructing. But that shouldn’t take away from the wonder of this story. Indeed what Bioshock Infinite really is is a Greek tragedy on the scale of the multiverse! In those old Greek tragedies the hero would met his end through his personal tragic flaw. It wasn’t necessarily good or bad that this end was met, simply necessary. All the infinity of universes with Booker, Elizabeth, Comstock, and Columbia, all the infinity of combinations of particles and rules, is our Greek hero. And they were all both brought into existence and simultaneously extinguished by their fatal flaw of Booker’s choice on that day of baptism. This flaw, this tiniest grouping of particles undergoing a tiny change in their position, changes drastically and forever an infinite number of universes and forces them into a gigantic act of self-destruction.

So to sum up, yes Booker will always choose to drown in that river with the same inevitability that that which cannot, exist will not exist."