Microsoft say minecraft is a 100 year old game, that they want it to be around for this long. Is this correct?

That isn't true. There's no way that a game would be supported for as long as 100 years. Minecraft is 9 years old so it's doing well to still be receiving updates, but I think they may stop updating minecraft by 2022 or a bit later, depending on whether minecraft is even remotely popular by then. Right now, I think fortnite is the reason why minecraft has lost popularity.