It really doesn't matter if you have an ability that lets you engage with the social pillars or not. The guy with the greatest likelihood of success will always be the first to step in. But they have a chance of failure.
They can't actually build to be good at it, in default 5E though, unless they have rolled stats and CHA to spare. Assuming they need STR or DEX and CON as primary and secondary (and they pretty much do), with other stat methods, whatever is left for CHA is likely to be pretty low, and then all you have is maybe Persuasion and one of Deception or Intimidate.
And any CHA-based character is going to do nearly as well untrained, and better if trained.
Uh-huh, but the point is, under D&D 5E's actual rules, the foppish Bard is going to have a better chance than the grizzled Fighter. In a lot of other games, that wouldn't be true. In some, it would even just matter mostly whether you were grizzled or foppish, and your "class" or equivalent would be irrelevant.
Not really though, because 5E is a party-based game, and in most parties, some other PC is going to be better than you at social stuff. To compete you'd have to make CHA your primary, making yourself actively worse at fighting. And 5E encourages you to have the highest person roll, and another assist - and you gain zero benefit from the person assisting actually being good at the skill in question.
I've considered giving fighters an extra skill proficiency as a way to deal with caster dominance in non-combat scenarios,
The problem is not Fighters. It's not a problem with Fighters, Rogues, Clerics, or Wizards. The problem is not really with classes.
The problem is that there is exactly one social attribute: Charisma. Every social skill keys to one attribute -- with the exception of Insight, which is so vaguely passive that it's essentially only useful to oppose Deception. That's the first part of what's wrong. Charisma carries virtually the entire social pillar on it's back.
Then, for the second part, there are several classes whose primary combat attribute is also Charisma. That's an absurd design that exposes the skill design flaw. It means that Bards, Sorcerers, Warlocks, and (to a lesser degree) Paladins are double good at all social interaction for free.
Why? Who thought that it was a good design idea to make a third of the classes head and shoulders better at all social interaction? They didn't design a third of all classes to be head and shoulders better at all combat encounters. Why are social encounters different? Why is the entire social pillar warped so badly?
The only reason this doesn't come up in actual play is because most tables don't use Charisma or social skills for every interaction. Instead, most tables roleplay things out, and the results are based on the social skills of the player and not on the abilities of the character at all.
Clearly Ged/Gandalf/Merlin/etc. aren't Wizards, by this logic
I wonder if it's worth investing in social skills as a fighter when there will certainly be one or more other party members who are better at it by virtue of being what they are. Unless you happen to be separated from your party, using your social skills would essentially be insisting that everyone accept a lower chance of success.
You don't get to use the barbarian's hit points, though.
If the sorcerer has a better persuasion mod, why not let them make the persuasion checks?
While I agree in part, it would be nice if there was a mechanical bonus to help offset the Cha casters' inherent advantage.
I get what you are saying. I would argue though that the fighter has unique "skills", specifically action surge and subclass abilities which are some of the best abilities in the game.I'm not sure about the social part, but in my idea, a class should be either mystical or skilled, with archetypes blurring that line. Classes that do not gain magic as part of their main features should have more skills. Not necessarily expertise, mind you. Just 1 or 2 more skill proficiencies.
Think so???Since we are talking about fighters lacking skills, I do feel the need to point out that a human fighter is the only character RAW that can be proficient in 17 skills by level 8.
Sure. A few have not placed charisma as their dump stat and gotten proficiency with social skills...........just like a 5e fighter can.Plenty of world leaders have historically come from the military, and charisma/social skills are certainly useful to them.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.