The longer you live, and especially the longer you remain healthy, the more evolutionarily fit you are. At least insofar as it doesn't funge against other traits evolution cares even more about, there's every reason to think evolution has optimized your machinery for longevity. We might find the non-helpful side of the tradeoff to actually be beneficial to modern human eyes even if they're horrible evolutionarily, like an ideal one would be "doubles your lifespan, makes you infertile", but there won't be anything evolution would see as a free lunch or a good tradeoff.
CellBioGuy attributed the "cancer licked in a decade or two" prediction to Thiel, not Kurzweil, do you actually have a source for it from Kurzweil? And does he have any particular reasons for stating it? Because even as someone on board with the singularity thing, that sounds like an insane pipe dream.
A couple of remarks to expand on RowanE's points for anyone who may be skeptical that evolution cares at all about longevity past (something like) typical childrearing age:
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