It is not. The fighter remained the most popular class in the game, even when it was out-damaged by Raging Barbarians, CoDzilla, and a host of other things.
The mechanics of the fighter have changed, sometimes radically, in each edition, yet it's popularity has been a constant.
It is the familiar, relatable concept of the archetypal hero, that's behind the class's enduring, Class-Tier-defying popularity.
And that was very much about mechanics and class balance. 4e eliminated LFQW and made classes better balanced than ever - if stii far from perfect, with fighters still sucking out of combat, and casters still having a non-trivial edge in versatility in & out of combat - and was marked for death by a segment if the fanbase that would not tolerate that, touching off the edition war.
5e returned to LFQW and more moderated (or at least, obfuscated) caster superiority, and is permitted to seek new players in relative peace.
Thing us, PF never attracted many new players, it attracted resentful D&D players.
No non-D&D TTRPG has ever attracted a lot of new players to the hobby - the closest claimant might be Storyteller in the 90s, but it pulled in new players on the LARP side, where it was dominant.
And the supply of resentful D&D players is pretty limited.
Ouch.