You seem to have two objections to cryonics:

1. Cryonics won't work.

2. Life extension is bad.

#1 is better addressed by the giant amount of information already written on the subject.

For #2 I'd like to quote a bit of Down and Out in the Magic Kingdom:

Everyone who had serious philosophical conundra on that subject just, you know, died, a generation before. The Bitchun Society didn't need to convert its detractors, just outlive them.

Even if you don't think life extension technologies are a good thing, it's only a matter of time before almost everyone thinks they are. Whatever part of "humanity" you value more than life will be gone forever.

ETA: Actually, there is an out: if you build FAI or some sort of world government and it enforces 20th century life spans on people. I can't say natural life spans because our lives were much shorter before modern sanitation and medicine.