In this case, yes, I wouldn't take it.
It's a misleading question. He wasn't "offered" 2.5billion dollars, he sold his company for that. 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.
If I think the $2.5 billion (plus four years of interest) is more than I can make over the next 4 years of not selling the company, I would have definitely taken it.
What is the value for the company itself? 40 employees - they're welcome to join me at my new, $2.5 bil funded, company. The brand name of 'Mojang'? Not worth keeping. Minecraft itself - very nice, but since Notch is already bored of working on the game, and was working on other games anyway, before getting bored of those, then the personal value of owning the brand 'Minecraft' has less emotional significance.
Sell, and rebuild for fun or profit. Either way it's a win. Either that, or take the Minecraft brand and build a bigger business ontop of it.