Speaking to D&D specifically, no matter how popular it is, it's going to be replaced by a new edition eventually. Wizards of the Coast will want to sell us all new books, and once sales dip to X level, the stars align, the prophecy comes to pass, or whatever metric they use to prognosticate these things reaches a given value, they will unveil the next big thing. It could be 2 years from now, 5 years from now, or 10 years form now (I'm going to be cynical and assume sooner rather than later, but it's not like I have special knowledge on this topic).
When this occurs, should the new game sell more product, does that mean the new game is simply better? Does that mean 5e was less of a game because it didn't make as much money?
Do we take market trends or inflation into account? A rising population?
Would 5e, compared to this hypothetical edition be said to have "failed" because it didn't make as much money?
Obviously not. But in this grim future, will people who were perfectly happy with 5e and have problems with 6e, 25e, Super Advanced Dungeons AND Dragons ("ampersands are so last century!") Mega Edition, or whatever they call it be told "there is nothing wrong with the new D AND D because it's making more money"?