If they're losing money on adventures I can understand ceasing production. But even if they're not the most profitable product, they provide a relatively inexpensive proving ground for new writers, new ideas, and new intellectual property that might still be in use decades later. Pathfinder started out as a series of adventures, and there's no reason WotC couldn't possibly develop their own thing as well. WotC is still making money off of adventures published by TSR more than 40 years ago. Far be it from me to pretend to know the market better than WotC though. It's entirely possible there's something I'm missing here.