Fighters do not need abilities or options other classes do not have in order to contribute outside of combat. The fact that other classes can also use the same techniques in no way affects the use of those same techniques by a fighter.
They don't need anything outside of combat that makes them uniquely capable in order to contribute. All PCs can contribute, regardless of class, outside of combat even if they are not the best theoretically possible.
All PCs can contribute. All have ability scores, 4 proficiencies, a background feature and a player with a working brain.
All classes have additional features that allow them to contribute outside of combat.
The suggestion is to give the fighter class options to bring their non-combat options closer to those of other classes.
If the group is trying to get out of a burning building because the wizard cast fireball in the living room and the door is stuck, that fighter with their +10 athletics is going to look pretty good compared to that bard with a +3 athletics. No one is saying that a fighter will be absolutely the best at any given skill. That doesn't mean they can't still contribute. Can they compete with a class that sacrifices combat ability, who's primary role in the game is to be the party face, in a persuasion check? When that fighter has a 10 charisma? No. That would be silly. Just like asking that bard with an 8 strength to open the stuck door with an athletics check.
It's like saying that Clydesdales are worthless because they can't win the Kentucky Derby. Meanwhile if I needed a field plowed I know which one I would pick.
The fighter is still having to roll for it. The bard might have the option to teleport or phase their way out, Shatter the door to smash it, Animate it to open, Polymorph themselves into something with a better strength etc. etc.
Even if they don't happen to have a spell that could be leveraged in that situation, they still have better ability checks than the fighter does. Therefore even if they have a lower bonus for this check in this very specific situation leveraging the fighter's primary ability score, they have a decent chance at other options like diving out the window and landing safely with Acrobatics.
This is the heart of the issue: Much can be achieved with just creativity, a hammer, and a complete disregard for the Laws of God and Man.
Much more can be achieved with creativity, a hammer, and a toolkit full of more specialised tools. They can not only do things that no amount of creativity would allow the hammer to achieve, they can also be used with creativity to achieve much more.
And, at the end of the day, if you cannot use any of those more specialised tools, because you don't have the right ones, or you already used them, or you're saving them for something else . . .
You still have that hammer.
First, I appreciate how you word you response. I feel like you and I are alike in that we're here to learn from others. Others who may or may not share our approach to the game. I know ENWorld has improved my approach to DMing over the last 6ish years.
TBH, I typically don't agree with the poster you are responding much but... I think I have a bit of common ground with them on this topic so maybe this will be helpful for you in your desire to understand what is likely a similar perspective.
First, at our table, players are not limited in the types of actions that they can propose for their characters. If it makes sense in the fiction and if there is no uncertainty, the action just happens. A Fighter wants to parley with a lazy Guard even though they don't have the highest CHA(Persuasion) modifier in the party? Why not? This is a game of make believe and story telling, not just a game of beating random numbers. (Note, I'm not trying to belittle anyone here who leans heavily into the crunch. I'm just saying it's not the only factor some tables find important for fun at their table. Sometimes actions can just succeed and we move on to the next challenge. Indeed, this is the approach of the Middle Path as detailed in the DMG pg 236)
Now, let's say this guard is a real stickler for rules and won't just let anyone past... but our 3rd level Champion Fighter has the Noble background and so has proficiency in Persuasion. With their 10 Charisma, our Fighter has a +2 to Persuasion. Okay, that's not as good as the Bard's +6 or the Sorcerer's +4, but this guard happens to be a social climber (or perhaps honors the social pecking order or...). The DM awards the Fighter with the Noble background Advantage on the check to try to sweet talk his way past. So... I guess where I'm going with this point is that checks don't happen in a vacuum. The DM creates these scenarios and presumably gives the NPCs Bonds/Ideals/Flaws/Motives and players, via their PCs, can try to discover those characteristics before going in for their ask. Maybe instead of a social climber, this guard is greedy or gullible or susceptible to name-dropping or... something else. At the end of the day, the Bard, despite their +6, just may not be the obvious or only choice to try to Persuade the guard. It might just be anyone in the party who slips the guard a gold piece or mentions the name of the local pit fighting champ or whatever. The DM is a major factor here in how they set up such scenes. Math and probabilities need not be the end all and be all in this game we all love.
It's also worth noting that the d20 is a fickle beast. The Bard being 20% better than the Fighter at Persuasion doesn't really mean a heck of a lot when that roll might come up once or twice in a session. It's just not noticeable with such a small sample size. Sure, I'll admit, the Bard typically does have a slightly better chance at succeeding if both PCs are utilizing the same approach to achieve the same goal against an agnostic target over the course of a campaign, but does that mean the Fighter lacks any value in the social interaction pillar? It seems like some here are lamenting that. Our table just doesn't worry about it. Oh - I suppose it is also worth noting, for those unswayed by my desire to downplay the math: failure can be fun.
The issue there is that unless the DM lays a fairly heavy thumb on the scales, the Bard's background is going to be the one that is useful an equal amount of times as the Fighter's. And there will be a fair amount of the time when neither can be applied and a check is called for, which is where the the bard having a much greater total of their modifiers to ability checks will mean that they will generally be better at those. Plus options like disguise or alter self to look like people the guard would be willing to parley with, detect thoughts to give an idea of what approach would go well, suggestion spell etc if the bard has them.