but even a slim chance of living for a very long time is worth almost any degree of suffering

Huh? Are you applying any discount rate to the value of living a very long time? The tradeoffs you are describing sound like they are calculated with the current utility of a very long lifespan being almost unbounded. For someone with a discount rate of 1% annually, an infinite lifespan has a net present utility of 100 years of lifespan. If, for instance, there was a 0.1% chance of the conjunction of a cure and an indefinite lifespan it wouldn't be worth -0.11 lifespan-years of utility, and a miserable two months could easily match that.

Also, as a transhumanist, I don't accept that most of us are eventually going to die of illnesses with fairly miserable end states.

It isn't desirable, of course. Nonetheless, looking, for instance, at the rather modest progress since the "war on cancer" was announced 40 years ago, it seems like a plausible extrapolation. Of course, a uFAI is perhaps plausible, and would technically satisfy your claim, a paperclipped population doesn't get cancer, but I don't think that is what you intended... What do you intend, and what is your evidence?