But any emails sent to my old address would be read by the new owner.
The web runs on the assumption that you have access to an email address and you'll never lose control of it. Ignoring that assumption and opening up your old users to identity theft just because you want to reissue short usernames that will again be squatted on is kind of crappy.
Maybe I'm just being naive, but it seems to me that someone getting a letter addressed to you is much less of an issue than someone getting your email.