I love the fact that a fighter is a blank slate as a class. It's traditionally the most open ended class, being someone who is good at fighting. If they'd attempted to create the subclasses as Shock Trooper, Fencer, Phalanx Soldier, etc they would have needed dozens of flavoured classes to cover even a fraction of the archetypal fighters.
I like the fact that the subclasses are distinct purely on a mechanical level, so I can draw on the vast array of fighter powers to build the PC I want, instead of it being proscribed for me.
I think there's a lot of truth in this. Classes like Warlock need a lot of flavor because, well, what is a warlock exactly? Players don't necessarily have a whole lot of fictional archetypes to draw on that fit the character build, so it helps to have some provided by the PHB.
The Fighter, on the other hand, fits as an archetype for a HUGE array of fantasy (and non-fantasy) stories. It doesn't need to be loaded with flavor, because players will probably be able to look back on something they read or watched in the last year and say, "well, that character could work as a fighter."
I don't think Mike Mearls is necessarily
wrong on this, because it's really hard to tell what the team would have come up with if they had taken a more flavorful route with the fighter subclasses. Quite possibly, it would have had changes that rippled out into other areas of the game. 5e seems to have been walking a lot of razor edges in terms of balancing simplicity and complexity, flavor and open-ness, etc. I also know that Mearls wasn't the only voice in creating D&D, and just because Mearls thinks another (undiscovered) solution might have been better, doesn't mean that the team, as a whole, failed in their execution of the game. There were many authors to D&D, and Mearls has said in the past that the team frequently chose to abandon ideas that he was fond of, in large part due to feedback
from the fans playtesting the game. Maybe this would have been one of those areas as well.
(I'll attempt to use a film-making analogy, though the analogies are often not so apt when comparing to RPG design, but it's the creative process I know best. When a director finishes a "director's cut" of a film, a lot of people watch that film and give notes and critiques. Sometimes, the director agrees with those critiques and tackles them with relish, but other times, the critiques are in conflict with what the director wants. These can come from anywhere — producers, distributors, editor, close friends, what have you. The director makes a case for her own solution, but the director is sometimes overruled. In this case, a director might just say, "well, serves you right," and execute the letter of the note. Usually, however, the director takes the external note and goes about figuring out a way to incorporate it into her own vision for the film, possibly with radical effects that ripple through the rest of the film. At the end of the day, the director might still say, "I wish we'd done it my way," but the final product is still her's, as much as a film made by many can ever be authored by one individual. This push and pull, and response to outside criticisms, limitations and edicts, is an essential part of the creative process, and often, though not always, makes for better films.)