It won't be forever. I remember Musk saying awhile back that they were going to flush inactive accounts at some point. I agree with this. Why should a username remain reserved if it hasn't been logged into for years?

Mainly because of "Login with Twitter". There isn't a proper way to tell downstream systems who have authenticated against Twitter that "The account JohnDoe is now a different user than they were."

Basically the same principle used to hijack accounts by buying an expired domain that had email addresses associated with it.

probably because my username is my first initial and my last name, and I don't really want someone impersonating me on a platform that I've used as a key social media presence since 2007?

If it's a key social media presence for you then that means you're logging into it and using it, so there is nothing to worry about.

Right, but if I delete my account, and someone takes my username, people who have known me over the past 15 years will probably think that's me (and whoever takes the username can very easily exploit that fact). Username reuse enables impersonation.

I have no idea if I still have my Myspace, Tribe.net, etc. accounts. Someone else might’ve taken them over by now for all I know and for all I care.

A fine approach. For me personally, I decided to delete my account so I wouldn't ever be tempted to log back in.