Sure, cryonics would help. But it wouldn't be more than a drop in the ocean. If QI is true, and cryonics is theoretically possible, then 500 years from now there'll be 3 kinds of universes: 1) Universes where I'm dead, either because cryonics didn't pan out (perhaps society collapsed), or because for some reason I wasn't revived. 2) Universes where I'm alive thanks to cryonics and 3) Universes where I'm alive due to quantum fluctuations 'miraculously' keeping me alive.

Clearly the measure of the 3rd kind of universe will be very very small compared to the other two. And since I don't experience the first, that means that subjectively I'm overwhelmingly likely to experience being alive thanks to cryonics. And in most of those universes I'm probably healthy and happy. So that sounds good.

But quantum immortality implies forced immortality forever. No way to escape, no way to permanently shut yourself down once you get bored with life. No way to die even after the heath death of the universe.

No matter how good the few trillion years before that will be, the end result will be floating around as an isolated mind in an empty universe, kept alive by random quantum fluctuations in an increasingly small measure of all universes that will nevertheless always have subjective measure of 1, for literally ever.

Now personally I don't think QI is very likely. In fact I consider it extremely unlikely. All I'm saying is that if it were true, that'd be a nightmare.