Content-addressed storage and error correcting codes mean that one can make bitrot astronomically unlikely with honestly minimal infra investment.

It's copyright that causes anything to disappear from the web IMO -- torrents never die.

EDIT: I am aware that unseeded torrents do in fact die. But it really doesn't take much to seed a whole hard drive's worth of rarely requested data -- this also detects bitrot and so corrects errors automatically if you're not the only copy.

If you are, there's ECC, as well as making another copy.