And if we are being honest, half of the "D&D only" folks don't actually know the rules to it, either. There are many reasons why GMs are rarer than players, and willingness to learn "all" the rules is one of them.Possibly, but it doesn't end at D&D. How much do the PbtA people want to learn systems not based on it? How about BRP people?
And I don't think that's entirely legit anyway; someone may be willing to learn a few systems but not an unlimited number of them.