I think the worst that could potentially happen is a repeat of Pathfinder being spawned by Paizo at the release of 4e. Lets remember that did not kill D&D either.
From WotCs perspective, worst case scenario is a 5e clone hits the shelves and they have more competition than they did before the OGL 1.1 hit. Then VTT people come up with tools that work even better with that than WotCs in-house thing does with OneDND.
From the perspective of the rest of us, all things considered, that's not a bad "worst case" to have at all.
But even if that happens, D&D will continue in some shape or form. They may scale back even more on the studio itself and produce only a handful of books, they may knock their VTT idea on the head (once again - anyone remember the previous times it was going to happen?), but the license is bound to be used in other ways that bring money in even if the RPG itself doesn't do well, even if it's just licensed properties.
Possibly, they'll put in extra effort to regain any lost market share, and customers everywhere win.