Over the years of using MN I've had a few email accounts, and have encountered so many problems when I forget their log in details. This means that I then can't access my MN account to change email addresses, etc, so have had to reregister. Of course this means that my usernames are unavailable so I have to think up new one which I always find hard.
I suspect there are many others in the same situation, and many unused accounts and therefore usernames. With that in mind, can accounts have an expiry so that if they are not used for a certain length of time then the account holder is contacted and asked to confirm they are still active. After all, MN only exists because people engage in threads. If inactive accounts are deleted and the user then decides to post after a long period away they can simply reregister. If accounts are deleted because users can no longer access their registered email address then that's unfortunate and they just need to reregister, but at least it means their old username is then available to them (or anyone else).