Even if cryonics work, won`t everybody die anyway, eventually?

You can make a general theoretical argument for that, based on entropy and the heat death of the universe. But this still allows us all to live very long lives, potentially longer lives than the life so far of the universe since the Big Bang. (And it's so far off that I wouldn't be too confident of our theoretical predictions either.)

If you mean that somebody dying of cancer now will still die of old age after they've been thawed, then this is a risk, but then you simply freeze them again until there's a cure for old age. (And people are working on that.) Remember, you only have to live long enough that medical science learns to extend life faster than you age.