We won't see a 6th edition until at least 2030, probably more like 2034 to coincide with the 60th anniversary. The current D&D team isn't large enough to design a game in anything less than "several years." Given the glacial pace of new content for 5e (which, yes, I know some like it, you don't have to remind me of that), they can probably spool things out for at least another 6 years or so before enough people get antsy, and they can do an interim 5e Revised to fill the gap if necessary.
In fact, I suspect they will at some point in the next six to eight years. They'll have enough content published that they can go through, touch on all the places they weren't happy with like Fighter subclass blandness, Beastmaster Ranger, Bladelocks vis-a-vis Hexblade patron, Sorcerer subclass stuff, etc. Further, they'll be able to put out content "faster" because it's all theoretically just updates/revamps/streamlining, keeping the brand fuelled long enough that, if something like a movie deal/TV show/whatever has gone well, they could persuade the corporate types that an investment in a new edition would drive sales and work well as a promotional effort for future entertainment media beyond the tabletop.