Personally, unless you're a bond villain, I really don't think anyone needs that kind of money. Notch was already a multi-millionaire, he had more than enough money to live well for the rest of his life without working another day.
That's not what happens in these cases. /Mojang/ was bought for $2.5 billion, not Notch. Share holders such as they are certainly get a healthy chunk of that but these kinds of agreements generally stipulate that the purchasing company invests much of the purchase amount into the purchased company. In other words, on paper Microsoft bought something for $2.5 billion but the actual cash that trades hands is far less. And it's very unlikely to be cash but rather other non-liquid assets like stock.
Then let's not forget taxation, which may well be part of why Microsoft put forth such a high dollar mark. There's business reasons to spend money you otherwise don't need to. Remember when Microsoft bought Skype for a mere $2 billion and everyone lost their minds?
There can also sometimes be advantages to buying a company that's operating in the red and liquidating it right after (not likely what's going on here). Buy a company, inherit its debt, pay less taxes that period, liquidate acquisition, settle its debt, move on. Rememebr when EA bought <insert game studio> and shut them down right after?
Most likely Notch personally is only a few hundred percent richer than he already was. The vast majority of that $2.5 billion is never going to go anywhere near his personal coffers.
