I think way way down the line official updates will stop, but the modding community will still be active. Might even get to the point where Mojang might just leave all the new ideas and updates to the community and then package them all together, similar to what Valve did for TF2 (just hopefully in a better direction than where they took it). I think the fact that Minecraft being "simple" in a way lends it to longevity; like a box of legos, it's easy to bring em out when you remember it and still have just as much fun as you did when you first got it.