I used to be a forever DM, but because I wanted to - and from many player stories I'd heard about other DMs, didn't trust other people to run the game. Though, every once in while I did get to do other games than D&D. I don't fit Matt's definition of "Forever DM", but I still feel the title applies to me.
Things did change during 4E, I gave up the reigns and let other people run games (non-D&D) and let myself be a player. It did make me reconsider some of my own DMing style.
When 5th rolled around, I went back to being the Forever DM, but about 2 years ago I stopped running again to be a player for a bit. I find now that while I like occasional DMing, I'm more interested in one-shots or short campaigns of a year or less. DMing is a lot of work if you let it be, and I'm too obsessive about RPGs not to end up neck deep when running a game. So, I just try and take smaller bites these days and mix playing vs. running when I can.
Moreover, I've started looking at doing solo RPGing, so I can actually play/run those games my other players aren't interested in trying. Right now, that means a Star Trek solo game, and afterward I'm hoping to do a Twilight 2000 solo game.