This is a thorny topic and really depends on what you mean by "usable" and "lost to accidents". Of the oft-cited 21 million BTC total:
I guarantee you trillions of private keys corresponding to that address exist. Mathematically around 2^96 should exist by the pigeon-hole principle. So the probability that none do is so crushingly humungously vanishingly small that it makes creating a SHA256 preimage look like child's play.