The simplest is the theory that the universe (or multiverse) is Very Very Big.

Do you mean this in an Occamian way? I suspect not, but I think you should make it clearer.

Anyway, this is a subject I've actually thought about a lot.

A lot of people (including Derek Parfit himself) think continuity is absolutely essential for personal identity / selfhood, but I don't. I've had such terribly marked psychological changes over the years that I cannot even conceive of the answer to the question, "Am I the same person as Grognor from 2009?" being yes in any real sense. I interpret this experience, along with of course the actual evidence from neuroscience, physics, and good® philosophers like Daniel Dennett to mean that personal identity doesn't really exist and is really just a sort of cognitive illusion that I don't understand and neither do people smarter than me.

That said, here is an excellent essay by reductionist atheist Occam's razor-wielding AI researcher Paul Almond arguing that there is no continuity of self. It is a good essay, but sadly it is a very boring essay.

Oh, and I'm not so sure about your assumption that personal measure should not be taken into account in determining whether to purchase cryonics. It's quite rational to maximize the probability that things as similar to you as possible exist, so that as many of them as possible get to count as "you" in whatever sense matters to whatever it is that does the decision making (tentatively, I'll call this "you").