Well that's not the fighter's fault as a class if you don't use the guidelines.
Nobody said it's the fighter's fault. That would be adding insult to injury. Like saying to the kid without a bike, it's too bad your parents just don't love you enough to get you one.
It is actually a system fault for using assumptions that in normal play don't pan out. Like giving the fighter his "keep up with the Joneses" stuff starting at 11th level, when not many games make it that far - by their own statistical data. By that time the Joneses have already moved from the neighborhood.
If you actually follow the guidelines, the fighter is the best at combat. If you don't follow the guidelines, the fighter is hurt but then it is up to the DM to adjust the game.
Guidelines are exactly that, guides. It would be good if the guides used normal play as their basis. There is a reason 6-8 combat encounters in a day don't happen that often in normal play. Since they don't happen that often the guides should have used the norm as the basis for evening out the playing field, not the other way around.
You must have 6-8 combat encounters so that everyone gets a chance to shine does not seem much like a guideline, but more like a requirement for the game to even out.
If you only have 1 fight a day, the base sorcerer is straight up overpowered and the bard is the weakest caster.
But if you have 3-5 in a day how does the sorcerer then stack up?
If the group is only doing 3-4 encounters between long rests and the group has a fighter, the DM should adjust the fighter. double all short rest features or maybe just action surge.
Sure, but it still does not speak to the real problem. The baseline is skewed in a direction that normal play does not usually fall into. Like having the real good stuff starting at 11th-17th level, a level that a large amount of groups will probably never play at.
But you can't say "I'm not following the game how it is designed and the base fighter doesn't work now."
I think that what is being shown is that the game was not necessarily built with the robustness necessary to allow the fighter to shine in his expected area of expertise, when normal play comes to the fore. If 6-8 combat encounters a day was the norm, without the DM having to make any adjustments, then I would agree with you. But 3-5 encounters is what we have been seeing for a very long time now, and that is the norm.
The game should play at its best within the norm (levels 1-10, 3-5 encounters/day, etc.) It should not require additional adjustments to classes, adventures, rest periods, or encounters based on assumptions that don't seem to occur often. The guidelines should take the norm as their baseline. And when the guidelines suggest different approaches (variant rest periods, etc.) they should warn about possible consequences (impact to rest period dependent classes).