At this stage, it looks like no edition of D&D is going to ever beat the sales of the original, since all editions have been a bare fraction since. I mean, Diablo vs Temple of Apshai sales numbers difference. With 5e, they managed to get some of the old 2e people to come back and buy new stuff, and that boosted sales, but it's still the same old market where every year more people are leaving the hobby than are coming in. The advent of online stores has allowed a lot of companies to cling to a kind of half-life, where collectors buy pnp stuff that they will never use - much like Steam is for computer games. Everyone I still talk to in the industry talks about it as a readers market, not a players market. And at the same time, the last remaining major companies like Hasbro/WotC have gotten better about reselling the same product over and over to the same people, but a lot of the old dev hands got out while the gettings good.
Unless Generation Z starts picking up pnp (as in buying product with their own money, not just playing with their da), there's a lot more shrinkage to come, too. So, no, there aren't really any huge fanbases to draw on, like there once was with D&D, back when it was a million seller. At this stage, the synergy tie-in just isn't there, so there's no reason to give the first cut of your profit to a bunch of guys not doing any of the work making the game.