Why couldn’t the mods in their spare time 
email posters who haven’t posted for , say, 12 months, and tell them they will be de-registered. This could free up names.
Of course, this might drop the number of registered persons in half, thereby cutting the advertising revenue of the site.
samclem…hahaha. Our spare time?
Actually, this would have to be for admins only, as moderators don’t have access to this information. Every now and then I DO look at the newest registrations, and remove the idjits who register with obviously fake emails. I don’t know, maybe their back browser button doesn’t work or something. Anyway, I do free up some names that way.
Generally, we don’t free up usernames because it would be rather confusing for someone new to come in and start using an old name.
Any time that I spend doing this means that I spend less time on other staff duties, and frankly, I’m not inclined to do so. But if I ever DO find myself at loose ends, I’ll keep your suggestion in mind.