First, that 50 million number was vague, unsourced, and specifically referred to the entire lifespan of D&D, not current users.DDB has what? A few million subscribers? A good chunk of those are free accounts to get info, made a free account and just don't use it, or who use the DM's stuff for free. Meanwhile there are an estimated 50 million D&D players out there.
DDB is a very small fraction of D&D players. The vast majority are pen and pencil tabletop players. Saying DDB will make it easy is a Red Herring. The focus needs to be on the tabletop players.
Secondly, DDB has 15million or so accounts, and several millions of paid subscribers. And growing fast. You are kidding yourself if you don't think it's where D&D already is.
Edit: WotC aren't going to call it "OneD&D" either. They are going to call it "D&D." You can insist that it is really 5.5e or 6.0e or Strawberry Vanilla e or whatever you want.