That begs the question, then, of why WotC even needs to keep the TTRPG going, at all.
Maybe it doesn't? Or at the very least, it doesn't need D&D to be a money-making powerhouse. Which easily explains why they currently aren't rushing product after product out the door to try and cash in on 5E in the short-term.
People love D&D. Many people love 5E. Every day more and more people are introduced to the game. So there's no reason to shoot your load in the first 12 months of the game's existence when you don't need the money to survive. You can take your time... make the arrival of each new "expansion" book more of an event by there being six months between them... and keep the D&D name out there in the stratosphere. That way, if/when comics, board games, video games, tv, film, tee-shirts, toys, or whatever else gets produced, the D&D name on it still gives it meaning.