I have seen Fighters be mediocre, defined far more by what equipment they use than what class features they have. The class features aren't bad, but they aren't actually defining things. Conversely, I have seen Barbarians who were awesome regardless of what they had or didn't have--and who did not, as far as I could tell, meaningfully "fall off" after level 11. (IIRC that campaign went to 14-ish?)
As with a number of pronouncements you have made regarding the efficacy of various classes in both the short and long term, I simply disagree with your evaluation. Your evaluation makes the Fighter out to be some kind of demigod among classes, nearly unbeatable at everything it does, and my experience is very much the opposite--it can be easily outshone if someone puts in just a little effort. Likewise, your constant Wizard doom-saying is nothing like my experience of the game.
The only way I can see the vast majority of the complaints you raise is if you're playing with people who never do even the tiniest bit of thinking about character effectiveness. A scattershot Wizard, a scattershot Barbarian, a scattershot Ranger? Yeah, they're going to suffer because those classes reward careful consideration to varying degrees--it doesn't require system mastery or optimization, just putting a little forethought into what you're doing and why. Fighter, on the other hand, doesn't really do much of that--a scattershot Fighter will probably do better than nearly any other class because the core is robust.
So...is that the problem here? Are you presuming essentially all players never even think about trying to do better with the tools they have? Again this isn't "optimization", that's trying for being the best you can possibly be. I'm talking about "hmm, magic missile isn't very good damage output...I should probably try chromatic orb." Or "oh hey, Wolf Totem is actually REALLY useful since my party has lots of melee attackers!" That's not optimization, it's just actually caring about effectiveness.
Short-sighted chasing of year-1 sales isn't good. That's the problem here--for both you and WotC. That's precisely what I'm talking about when I mention things being sanded down so perfectly smooth, people slide right in....and slide right back out again.
As I said: Texture matters. Achievement matters. Depth matters. With no texture, there's nothing to grip people, nothing to keep them invested. With no possibility of personal achievement, with no development of mastery and no ability to see how you've gotten better at play, there's no ownership, no personal investment beyond the story of the character, and that story ends when the campaign ends. With no depth, the only thing to explore and uncover is the world itself--and that world ends when the campaign ends, at least in most cases. (I know there are exceptions, but those exceptions are themselves deeply invested players.)
Casuals are a huge source of revenue and absolutely should not be ignored; exactly the opposite, actually, they should be courted. But when you prioritize them to the exclusion of folks who like complexity, you gut the community. You remove one of the most important blocs, people who are deeply invested and proud of it, people who love to engage and speak out. The core of non-casual players may be only a smaller portion, but they form a critical structural component of the community.
If it lasts to 2029, it will have lasted five years. Hence...you literally repeated what I said, I would be shocked if it doesn't make it to mid-2029. I would be surprised if it doesn't make it to 2030. But I would also be (very!) shocked if it makes it to, say, 2032.
I expect internal efforts to make 6e kicking up sometime in 2028--when they can gauge the long-term response to 5.5e. I expect rumors to start circulating in 2029 or 2030, with an announcement following about a year after the rumors start.
