As others have already mentioned: for the most part, new editions are a good way to bring up sales again. And you gotta sell people something if you want to stay in business. In theory, you could also sell them adventures (5e had more success with this than I expected) or setting books, but, based on what I hear from publishers, people mostly love to buy books with new rules. So that's then your best option for good sales.
Now regarding the question whether it really needs full resets, that's becoming more tricky. I think, just like video games (and software in general), roleplaying games benefit from multiple iterations in which an existing design is polished. And in that regard, I think D&D would often have had room for more iterations of the same design.
However, eventually you will hit a point where improving the game is becoming harder and harder within the existing design constraints. And at that point a new edition really benefits from rethinking a game based on a new framework. Add to that the fact that preferences of the target audience might chance over time (more or less crunchy, more or less heroic, etc.). I think a publisher unwilling to reflect that will see their game become stale and slowly fading into obsolescence.
Now I personally would prefer if, instead of revamping the game under the name D&D, there would be a new game E&E (maybe Estates & Efreeti) that makes those changes and the old game would be untouched. I'm afraid that's not the way people with popular brands operate, though.