I am suspicious about this argument:

Cryonics (and the rest of biotechnology) can be used to make individuals live longer than political parties, governments, and empires. People who have outlived three governments won't take the fourth so seriously.

I think that if people will live longer, then also the political parties will exist longer.

In extreme situations, sometimes a dictatorship falls only because the dictator died of old age. Now imagine a dictatorship where the dictator has the best technology for themselves, and a mediocre technology for their supporters; and of course their enemies get none. So they will have like thousand years old vampires ruling the country, opposed by some young folks who may try some cool tricks, but the vampires have already seen all those tricks dozen times.

Would it be interesting to have Thomas Jefferson alive and hear his opinions about the world today? Sure it would be. But remember that on the other side of the planet you would have Stalin alive. (At least Hitler would be dead.)