Then one would think you'd remember.
There's "remember" and then there's "agree with you." The point you're trying to make is that 4e was somehow wildly unpopular with D&Ders, and it wasn't, indeed, you get into why, below...
I say 4e fans are a minority because I believe that any fans of a particular edition are a minority.
Fans of any particular edition, exclusive of all other editions, are certainly
each in a fairly small minority. I don't even fall into the category of '4e fan,' when you set that bar. I've been playing the game since 1980, and in that time, only the second half of the 2e run disappointed me, and I haven't ever entirely stopped playing AD&D, 3e or 4e, even though I'm running & actively promoting 5e.
Similarly, fans who specifically rejected one edition are clearly in a small minority. Every once in a while, edition warriors would acknowledge that there really weren't that many of us causing all the fuss, and then get back to it.
First and foremost they like D&D, play D&D, and don't care about the specific flavour of the game. (Which comes from the what Mearls reported from the surveys,.)
I hadn't heard that quote, but it's hardly surprising. Each new ed does very well for a while, then tapers off. It fits with a very stable fan base on the one hand, and a basic model that can only move supplements for so long before bloat starts to diminish returns, on the other. Past editions have each tried to do something in particular, well, often something the prior edition really flubbed. 5e finally got the bright idea to try to do everything past editions had done particularly well, at least, with an optional rule or few in place. They called that 'modular,' which conjures the wrong idea (and a much higher bar) in my mind, because of my professional background, but, looking at what they actually came up with, and how neatly in dovetails with the RAW-repudiating 'rulings not rules' maxim, it's surprisingly workable. It's just not there yet, and stuff modern D&D did well (both 3e & 4e) is where the most still needs to be done. Though, if we're being realistic, 5e can only ever get details of 3.5 (like PrCs, perhaps) & 4e (like the warlord) 'right,' it can never go back to the RAW-uber-alles system-mastery rewards of 3.5 or the balance-uber-alles structure of 4e, there's just no foundation for either - and 5e treats such player-empowerment schemes as antithetical to it's DM-empowerment mandate.
it really looks like 4th Edition wasn't performing as well as desired from very early.
There's no question 4e didn't perform as it needed to. The goals set for 4e were unrealistic, the entire industry has never even come close, RPGs, even D&D never did break into the mainstream like video games & MMOs have. D&D was given some serious resources to take a shot at an unprecedented level of success, instead of being dropped to some lower level of investment, which is where it is now, having failed to establish MMO-like revenue streams.
I take no joy in the events that led to the game being managed by a skeleton crew.
It's not a wonderful result for the business or for fans, but it's not because of the sound & fury of the edition war or 4e radically under-performing prior eds (or being so 'unpopular' that the it's innovations should be excised from the game forever), but because D&D took it's best shot, and couldn't expand it's appeal enough, even taking somewhat radical-seeming steps to break out of the box it'd been in.
Do they deserve to be excluded? Certainly not.
Then maybe we can get past this unanswerable edition war issue of relative popularity? It doesn't matter if your faux-warlord fighter archetype sold badly on DMsG, or that D&D didn't ever reach MMO levels of income.
But it's a pretty big jump from "don't exclude" to "make a class just for them."
It's really not. The Warlord has become something of a poster boy for 4e, pointedly excluding it creates an unsavory appearance of having taken sides in the edition war. Including it plays to the stated goals of 5e, not just to including the best bits from prior eds, but to expanding the range of playstyles it supports.
Couldn't they also honour 4e by doing an adventure storyline set in the Nentir Vale? Maybe some details on the Dawn War or maps of the 4e cosmology?
The World Axis cosmology was presented as an alternate, IIRC, and you can point to other bits and pieces here and there. You can also point to how higher profile aspects of 4e haven't made it over, and how those that have are marginalized or bowdlerized. So there's definitely more to be done on that front. But, no, the Warlord is the most pointed of omissions, and it's inclusion would help the game meet more of its goals, as well. If done well, it'd be an all-around win.