Minecraft the game does not generate a continuous income after the initial sales, but recently they started an official multiplayer server hosting service (called "Minecraft realms"). You can buy a private server for $10 per month.
Given that Minecraft is played by a lot of kids whose parents whose parents or themselves do not know how to set up a server, I think this is a very viable monetization strategy, albeit launched a bit late.
Of course, there have been 3rd party commercial hosting services for Minecraft for a long time, but again, it's not something kids and their parents go after.
Hell, I know how to set up a server but I am considering hiring a Minecraft realm for the winter months, just because it is less hassle than setting up a server and the price is reasonable.
In my own personal case though, the second I had anything worth $2 million I would sell out and retire. I just want to be able to feed myself and shelter myself and I'll gladly do independent research the rest of my life.
(I'm with you on "sell out at $2 million" in general, though. For the area I want to live in, which I coincidentally already live in, that'll last a lifetime.)
I can't imagine Bill Gates selling MS in '86. He wanted to build an empire.
I won't pretend that makes me really know him as a person but I can say I've never got the impression he cares much about empire and/or massive ego building. I would venture a guess that by now he accepted the fact that MC was his one massive strike and that he likely won't have another one & would be content to live his life of luxury with that.
I don't think he wants to build an empire, but he wants the freedom to do his own thing, with people that he cares about (the mojangstas).