While I agree with you to some extent, I believe I can play devil's advocate.

Though it's speculation, I would assign the probability of evolution continuing to work (and improve) on the human race as pretty high - what gain does the human species have in preserving humans from the 21st century indefinitely, when 23rd century or later humans are better?

Is it clear that progress through evolution is optimal? Evolution is insensible and doesn't consistently result in linear progress (e.g., we could be wiped out by a virus). In any case, relying on evolution is futile: we seem to be at a new stage of pattern formation where cultural and technical evolution is working at a far greater pace than generational, genetic evolution. Evolution, and any plans that it might have had for us, is falling to the wayside.

the necessity of refreshing the population of each species.

This is not to say that a 70-100 year lifespan is ideal. We spend so much of our productive lifetimes learning, it seems we could be much more productive if the working sector had more productive years before retirement.

Overall, in no way can I think of cryonics benefiting anyone other than the individual's (I think simply genetic) desire to avoid death

What is the value of the group if the individual doesn't matter? Death is something hanging over us that causes a lot of misery, fear and anxiety. Death is a very unpleasant consequence for sentient beings -- I would argue that it should never have been allowed to happen, this combination of sentience and mortality. Wouldn't it be good to fix that problem for all sentient beings, now and in the future?