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.
True, but it doesn't matter that there is an infinite amount of private keys to any given address. The only thing important is the fact that the probability of generating a matching private key for a given address is 1/[numberOfBitcoinAddresses]