To me, arguing that cryonics isn't worth it because humans can't really feel improvements in quality of life

You're committing fallacious reasoning here. Just because I'm saying "increased future happiness is not a good argument for cryonics", does NOT mean that I think there are no good arguments for cryonics. I just think that yours is not one of them.

I'm specifically critiquing your phrasing of "the payoff is in a different class to anything else". You can make some additional assumptions that lead to "Cryonics + Fun Theory Utopia", but it's fundamentally less likely to occur than just "Cryonics" by itself. Equally, million year lifespans are less likely than thousand year lifespans. Cryonics, by itself, is not in a different class; it's just waking up from a coma after years of being clinically dead.