In D&D, especially in later editions 3E and upward, everyone can fight anyway. A fighter class, at least the way it is used now, is thus not really needed. It can't be so much better in combat than everyone else because of balance reasons and it is very hard to find a non combat niche for a class which is so centered around combat than the fighter.
I think it would be best if the fighter class would be rolled into the existing classes, Ranger for skirmisher and archer, Paladin for knights and Barbarian for the front line fighter.
I can't be sure but you sound like your view of D&D is quite "gamist": maybe you assume balance has to be there in each pillar separately thus the Fighter (who has little out-of-combat baggage) is stuck at being less than the other martial classes; you also sound like you want combat roles covered, and that's it.
My view is very different, I care for balance either in the bigger picture across all pillars, or on the smallest scale with relation to specific alternatives (e.g. one option should not be
straight better than an alternative option
in every circumstance). Therefore I actually prefer the Fighter to be slightly superior to all other martial classes, if those have some out-of-combat edge the Fighter doesn't have.
And as for tactical roles, I don't care if two classes cover the same role or if a class covers multiple roles. On the contrary, I find it incredibly irritating when a game forces too much the narrative and the tactical role together instead of giving more freedom.
If there were 30 different specialized martial classes, maybe the Fighter would start being redundant. But 3-5 martial classes aren't remotely enough to cover most concepts of a martial character. There have been more times I wanted to play a martial character that was neither wild, holy nor nature-oriented, than the times I wanted to play one of those.
A game with 30 martial classes, 30 arcane spellcasters, 30 divine spellcasters and 30 scoundrel-types would certainly work fine. But so would a game with Fighter, Wizard, Cleric and Rogue, and nothing else. And since we're closer to the latter, it would be easier to get rid of Barbarians, Paladins and Rangers instead, and
no character concept would stay unsupported.
No it wouldn't. All those other classes come with baggage that sometime you don't want.
Exactly.