Now to negate that last line -
Can you imagine what would happen to the gaming industry if he did retire? So much of the money is made off technology entirely influenced by him, if not directly created by him.
Who would be filling those enormous computer genius shoes? Who would be building the engines used by some of the best (and worst) games out there? Epic? I hope not, I don't like the Unreal engine as much, and it's as good as it is mostly due to trying to one-up Carmack. The lithtech engine (or whatever they call it now?) Hardly.
And what will id be without their engine? All their games have been fun, but no has been buying them for their gaming aspects, not since Doom. Everyone buys them for the technology, the whiz-bang graphics and engine.
The man revolutionized gaming, and is still probably the biggest contributor to the industry. Retirement leave a lot hanging. Hell, it might lead to a decline of the FPS, much like Flight Simulators went from the dominant genre to all-but-dead (give me Red Baron 3, dammit, and make it decent, with clouds to hide in, not pixels like 2).