As the devs said at BlizzCon, we have a ton of stuff lined up for future expansions so you can relax :) We never said level 100 would be the end, and I think it's pretty clear that we still have many adventures ahead of us.
It's like saying "I really like my perfectly fine Mercedes but I have had it for exactly 100 days so I'm just gonna go and drive it in to this mining quarry".
