As I said above... Pathfinder was the exception that proves the rule. So many disparate things happened that allowed Pathfinder to not only come into being but also flourish, that I don't believe anyone actually thinks that will ever happen again.
I'd argue the exact same thing happened to Kobold Press who has risen quite well over the 5e time and now has their own flavor of 5e to build off of for as long as they choose. They're not as big as Paizo, obviously, but they seem to be doing fine.
Look, if you honestly think that people are going to stick with variant 5E rules if/when WotC actually produces an eventual 6E (which has been my point in all of this)... you certainly can do so. I myself would not take that bet however.
I absolutely believe people will stick to 5e if WOTC goes to a 6e (which isn't a forgone conclusion. Who knows what they're going to do hereon out). I know this because people stuck to older versions of D&D. The OSR seems to be doing great. Shadowdark made a million and a half dollars and has a strong community of people who play. Other old-school variants like Knave and Cairn and Dolmenwood did great as well. Dragonbane, an update to a Swedish version of D&D from the 80s, is an awesome and popular RPG.
I suspect when you say "people" that you mean "most people" or something like that – a number we can never actually know. No doubt D&D will likely remain
most popular but who cares? Why does that matter? We don't need to convince the world to play whatever RPG we want to play. We only need to convince four to six of our friends.
And if we want to help break the hold that D&D has on the hobby, maybe we spend more time talking to people about all the other games out there – not to take D&D down (I love D&D) but to show how wide the hobby can be.
It is my personal belief that D&D 5E14, D&D 5E24, Level Up, Tales of the Valiant, and all the other books (for players, DMs and all manner of monster book) built upon the D&D 5E engine will find their audiences fall off a cliff if/when a 6E comes into being because their success and popularity is due directly to WotC's D&D 5E's success and popularity. Everyone making their bones off of WotC's base game will see that well dry up once most players have the new game to buy. But that's the risk they take by following in WotC's wake. They can make their money while the 5E game in all its permutations has its followers and popularity... and have to deal with the consequences if/when those followers and popularity go away. And hopefully they have now prepared for that.
There are so many factors that go into keeping a business afloat that what WOTC decides to do with a 6e in some number of years doesn't even come into it. Now we're speculating about complete fantasy lands. None of us have any idea what the next five years hold for the hobby. I bet Kobold Press, EN World publishing, Free League, Arcane Library, MCDM, and others aren't worried about that right now. They're busy making awesome games.
If you believe otherwise... that's fine. Hopefully there will be pockets of players still playing 5E variants while everyone else has moved on to 6E. But if our 4E players here on the boards are any indication... those pockets of players playing an older game can be few and far between,
4e was far more fragile than 5e is. 4e had no SRD. It had no open license. There's no alternative compatible version of 4e. But MCDM made almost five million dollars building a new RPG that definitely has inspiration from 4e. 13th Age is an incredible RPG built by the lead designer of 4e and I much prefer it. They have a new version coming out next year I think.
But then again... knock on wood that'll be a question most of us will not actually have to worry about any time soon, because for all we know this 5E24 "keep the game evergreen" strategy might actually work. And thus D&D 5E (and all the other 5Es out there) won't have to see what a 6E would do to their market share any time soon.
I don't think any of us need to worry about what a 6e will do, if there is one. I think we have plenty of things we can do right now to run awesome games for our friends and help expand the whole RPG hobby to everyone else to enjoy it.
That's my charge, anyway.