It's a platform for development. More importantly, it's a platform that lets many people create things in 3D without having to bother with OpenGL/etc.
If you think I'm talking about pixel art or other static things, or if you think I'm talking about mods, I suggest watching these, which are all stock minecraft only.
* SimCity reimplemented in vanilla redstone & in-game command blocks *
http://www.jigarbov.net/simburbia-map-release/
* FVDisco's Cake Defense, a wave-attack game with many features. (everything FVDisco has done is outstanding...) *
https://www.youtube.com/watch?v=OwxD3axvgt4
* Razul, an older scripted rpg adventure game with command-block based quest system *
http://www.planetminecraft.com/project/razul---skyrim-inspir...
* Hypixel, a many-thousand player custom server that works with the stock client *
https://www.youtube.com/watch?v=k1BdSq8fMtY
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The larger ideas that Minecraft is effectively a prototype for is basically the VR environments we've wished for over the last several decades. We achieved a lot of the 3D graphics capability (and are ok with sound, the few cases where the developer bothers), but interactivity is just as important for immersion. Not being able to interact with things that are not part of the "intended" experience can rip you right out of your suspension of disbelief.
This interactivity is what Minecraft gets right, and we've barely scratched the surface.
Minecraft : interactive 3D voxel games :: NCSA Mosaic : hypertext documents
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).
Why take the big payment today when you can have the bigger payment on-going? Especially if you want to continue to make it your life's work. Look at some gaming stars from the past. John Romero didn't "make us his bitch" with his post iD offerings. Carmack's career seemed pretty down until his recent excitement with Oculus and even that happened while he was playing with space companies on the side. Even with all his brains and cred, he's now just a Facebook employee and his boss is a 20-something php coder.
A lot of gaming luminaries are often one-trick ponies. Its difficult to get lightning to strike twice. Smart money is keeping a tight hold of the IP that made you successful. Notch has seen what happens when you lose your IP and maybe he doesn't like it.