The "dead" accounts aren't as much of a burden as you might think. Unless those users had hundreds of gigs of stored photos, there's no significant disk space penalty. This isn't the era where it costs thousands to add a few gigs of disk space. An unused account has zero CPU penalty. Security--- well, perhaps. But you can simply lock anything that's gone idle over "x" time and require that user to send a request to unlock it were that to happen. None of the commercial car forums I'm on has ever deleted user accounts that went idle as far as I've ever seen. It's more likely the site will simply die and disappear along with all of the data vs. having a few thousand users get deleted.
Another downside of deleting users is that all former posts by that user now change to "guest" or possibly could even be deleted. If not deleted, it makes it hard to search for anything historical by any specific user.