You guys are focusing too much on the game. The Minecraft, as an icon, is valuable in and of itself. Disney has Mickey. Nintendo has Mario. Sega has Sonic. Microsoft could possibly be buying Steve. If Steve, as an icon, can last decades into the future, it will be well worth the price tag to Microsoft.
Think Nintendo has made at least $2 billion off of Mario? They certainly didn't make that much off of Mario Bros. or even Super Mario Bros., but has an icon, I'm sure Mario has been worth way more than that.