Finding a convincing reference class in which cryonics, singularity, superhuman AI etc. are highly probable

I'll nominate hypotheses or predictions predicated on materialism, or maybe the Copernican/mediocrity principle. In an indifferent universe, there's nothing special about the current human condition; in the long run, we should expect things to be very different in some way.

Note that a lot of the people around this community who take radically positive scenarios seriously, also take human extinction risks seriously, and seem to try to carefully analyze their uncertainty. The attitude seems markedly different from typical doom/salvation prophecies.

(Yes, predictions about human extinction events have never come true either, but there are strong anthropic reasons to expect this: if there had been a human extinction even in our past, we wouldn't expect to be here to talk about it!)