Those aren't my assumptions. Those are the assumptions given in the game where easy is described as 10 DC, moderate is described as 15 DC, and hard is described as 20 DC.
A +9 bonus from a combination of ability score and proficiency is plenty, and very high looking through the MM or adventure books.
The base mechanic is d20 plus ability modifier getting back to the premise that checks are inclusive for any PC to use.
Expertise is the exception, not the expectation, because it's a class feature rarely given out. It can be used for even higher DC's that rarely occur or offset lack of an ability modifier.
Just because expertise can exist doesn't increase DC's. Flat DC's almost never exceed 20 and opposed checks usually don't include proficiency let alone expertise.
It's the overkill/overhead principle applied to ability checks. It doesn't matter how much a PC exceeds 15 DC checks.
But you don't get a +9 until either level 9 (if you have a +5 mod) or level 13 (if you have a +4)
So, you are talking about the game being half over by that point, if not almost 3/4's over. Most PCs are only getting a +5 to +7 to their best skills for most of the game. Meaning at best, with your best abilities, you have a 65% chance of succeeding a DC 15 check. That is hardly reliable.
Strength and athletics were meant to be applied to exploration checks. You rarely climb or open stuck doors? Jumping? Swimming? Breaking objects?
Swimming and Climbing do not require checks except in extraordinary circumstances. They simply count as costing double your movement.
pg 182 "Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you’re climbing, swimming, or crawling. You ignore this extra cost if you have a climbing speed and use it to climb, or a swimming speed and use it to swim. At the DM’s option, climbing a slippery vertical surface or one with few handholds requires a successful Strength (Athletics) check. Similarly, gaining any distance in rough water might require a successful Strength (Athletics) check."
Same with jumping, if you have the clearance to move forward at least 10 ft, you automatically jump a distance equal to your strenth mod with no check. Meaning a 10 strength is enough to clear a 10 ft gap. High jumping allows you to jump 3+mod+1.5 time height (By putting your hands above your head, and therefore grabbing a ledge to pull yourself up, which would be climbing) meaning your average 5.5 ft person can reach an 11 ft high ledge with ease, and no check.
And when it comes to forcing open doors or breaking objects, there is usually no penalty to failing. I usually have to ask other DMs to stop just having us fail, because we simply roll again, and again and again until we succeed.
So, yes, the strength athletics rolls rarely come up at the table. We rarely encounter slick, sheer walls that are more than 10 ft tall or rough water to swim through, we rarely encounter gaps of 15 ft or more (and there is usually one strength character to clear that gap).
If you are using different rules, I could see it coming up more often for you. I have encountered DMs who insist on making us roll to climb cliffs, ropes, or trees, or jump even small 5 ft gaps. None of which is required by the rules as written.
Equipment would be climbing kits, crowbars, and vials of acid as examples. Other classes using them too is irrelevant. It's an option fighters have. Options only for fighters would be moving the goal posts from having options to having exclusive options.
Social equipment includes gaming kits, musical instruments, and other artisan tools. These are ways in to build contacts for those favor checks or directly curry friendly attitudes.
It isn't irrelevant though.
"We need to get this lock off, fast!"
"Don't worry I have a vial of acid to eat through the lock."
Which class had it? The wizard? The Fighter? The Warlock? The Artificer? Anybody could have that.
In one of my games, my rogue (who has a climbing speed) and the Monk (who doesn't need them) are just about to go shopping for Climbing Kits to give to our Sorcerer, Cleric, and Paladin. They will work the same for the sorcerer as they do the Paladin.
So, saying fighters can buy acid or buy a climbing kit is like saying Fighters can take Perception as a skill proficiency. So can literally anybody else. So if anybody can do it, what about that makes the fighter contribute to the party?
Yes. 16 strength or dexterity, 16 charisma, 14 constitution, and 12 wisdom. Or take the 14 Charisma instead and add an ASI later.
That's a half-elf so easy enough for point buy and extra skill proficiencies.
I don't need strength and dexterity on most typical fighter builds. Either / or works.
So, a specific race option, okay. Can't do that with most racial options though.
Also, don't forget that having a strength lower than 15 grants you a penalty on using Plate Armor, lowering your potential AC by a point unless you take a feat to compensate.
You can compensate, but it is just one more thing you have to do to keep up with expectations.
How does that assertion demonstrate a fighter is bad at combat by learning social abilities?
A 14 or 16 charisma does not preclude strength or dexterity.
So being on equal footing means fighters aren't struggling here either.
Then invest in it.
Well, a 14 means that their skill bonus is a +4, meaning that they have a 45% chance of success against DC 15.
Also, it does end up making it harder to invest in Constitution, Strength (for armor), Dexteritiy (for initiative and saves), and Wisdom (for spotting ambushes or lies)
So, investing has a cost. Sometimes to abilities that effect combat.
And combat is guaranteed to happen, while social aspects and rolls aren't.
The bolded part is where I think you are making a mistake. Those are predominantly initiated by an action of the PC. I take actions based on what I expect from my characters. A DM would have to take direct control of my character to somehow prevent that.
It doesn't matter if what I do automatically succeeds, or fails, or requires a check. I build to my concept and play to that concept.
If social abilities aren't prominent then session 0 should let's me know anyway. It's moot if I don't have to take abilities I won't need or use anyway.
sigh
Look, different tables have different expectations man. I literally can phrase it no other way, or explain it any other way.
We played the dungeon of the Mad Mage for a about a half dozen sessions. We never had a single encounter that required any persuasion or any other social check. There was a revenant, he said he wanted vengeance for the people who betrayed him. We said yes, and every encounter that didn't start with "monsters attack you" was "the revenant screams and gouges out the eyes of their target"
Taking any social abilities would have been a complete and total waste. But even the DM wouldn't have been able to tell us that, because if we had said no to the revenant, or we had encountered the other people first, then they could have come into play.
Another game, same group, actually literally the next game, was billed as a monster hunting game. Abotu two sessions in we got embroiled in poltical intrigue up to our eyeballs and needed a plethora of social skills to keep up with the plot. Again, no way of knowing, because the DM was very much a "I let your actions determine the plot" and by choosing to care about the city under siege, we got involved in fey politics.
So yes, I get that "the PC's actions determine what the PC does" but I can't choose to talk to people if no people are presented to talk to. I can't choose to roll skills, if everyone just gives us what we want, because the DM wants to get us out of the town already and into the dungeon, where those skills won't matter.
And if I end up in a game where those skills do matter, where social interaction is highly important... then I'm playing a charisma class that gets benefits and bonuses to explicitly that thing.