An username will always be available, there are just too many combinations possible.
However, the most common names, often the most wanted names, are already taken. I do agree that these names should be released, if an user has not been online for, let's say 2 years. Give him a randomized name in return, like user103230409.
But then again, this will never happen, so why bother.
I think it's the case that even though a chess.com member deletes his/her account, his/her username is still found around chess.com in the form of forum contrubutions (among other things). I also know that no two chess.com members can have the same username.
This leads to my question: doesn't that mean that people many years later who want to join chess.com cannot because there won't be any usernames left for them to use since they've gone extinct?
If so, how will this site continue to gain members in the future? Will the entire site have to be reset? Or will there be an "expiry date" for usernames of deleted accounts?