D&D won't be slain by MMORPGs. It will be found lying in a pool of its own verbiage with a Bananagrams bag sticking out of its back and coughing up Story Cubes.
The next big thing won't be D&D 5e. Very soon, it's going to occur to someone to do to D&D what Bananagrams did to Scrabble: take out the wait time and accounting, give it that new car smell, and market it to a general audience as if the game it's based on had never happened. Ever played Bananagrams? It's Scrabble on meth. There's no board and no math, and all players go simultaneously instead of waiting for their turn. Right now it's #11 on Amazon. Scrabble is #428. What's #1? Rory's Story Cubes, "a pocket-sized creative story generator" which also showed up in my kid's Xmas stocking. So, right now, people, go take D&D, remake it for the masses, sell it to Gamewright, and I promise you will make a fortune. Quick, before someone else does it first.