So, make the character on the front line more likely to die, and focus on... what? Wisdom and Charisma are only useful if you have... skills.
ABSOLUTELY. Do you want your fighter only to be good at combat or good at other things too? It is a choice you make and then live with it.
Also I have played numerous fighters with a 10 Con and played two of them with a 12 Con all the way to 20th level. None of my fighters have died in play (yet). I have had an Artificer, Monk, 2 Wizards, Bard and Rogue die in play.
I mean, sure, yeah, you can build a fighter who has less hp and instead has a high charisma, which allows them to actually succeed at persuasion rolls.
Yes you can. So if that is what you want to it.
... But they can't read minds.
Yes they can detect thoughts is available through a feat to any PC and also available to the Eldritch Knight subclass. Again choices.
They can't change their appearance with a single action.
Yes they can, many ways. Off the top of my head - Changinling, Glaysya Tiefling, Magic Initiate, Eldritch Knight, Shadow Touched.
They can't charm.
Yes they can - eldritch Knight, Fey touched, Magic initiate, Eladrin, Fierna Tiefling
They can't suggest courses of action.
Any character at 1st level can suggest courses of action. I don't understand this.
OR Wisdom, by which you are likely focusing on perception. But... while perception is useful, what can the fighter do that is equal to detect good and evil? Detect Poison? Detect Magic? See Invisibility? True Seeing?
A fighter can do all of those things except true seeing if you make build choices to enable it.
Most people aren't stupid when they build their characters. They give front-liners high Con because they get hit the most, and they don't want their characters dying.
And don't want their characters being good at other things. You do not need a high constitution to avoid dying. It does give you a buffer, but it comes at a cost in other pillars. If you want to pay that cost fine, but that is a purposeful decision.
But even if a fighter puts a high score into wisdom or charisma... they are still limited. They are limited to the skills and what the DM allows. They can't break out of that.
Sure. The DM has the ultimate power and say here.
What if, by level 10 or so, a martial character with Investigation could roll and on a success, they discover enough clues to actually get to "see" what happened in a room? The DM would describe to them, for example, that two men slipped in through the window, startled Mr Darcy, who ran for his sword, but he was hit by a thrown dagger with an ornate handle. The two men dragged him back to his chair, where the second used magic to seal the wound, and make it appear as though he died of natural causes.
This is something that absolutely happens in fiction, with skilled investigator characters, and if the player knew that, they might be far more interested in putting a higher Int on their character, and building towards that ability.
OR, if they want to play the jock fighter, maybe give their athletics a way to destroy scenery more easily, or an ability to ignore environmental damage and effects through sheer toughness (Oden standing for an hour in a pot of boiling oil to save his men from execution)
There are things that would allow us to compete or exceed spells, things we lack, and can't build towards.
Why can't you do those things now?
Should be less effective? No. But at that point, are you really going to say that catching someone on fire is should be less effective than shooting them with an arrow? Taking Firebolt is a decision that has a real, tangible cost too.
It depends on the fire. IRL getting shot with an arrow is generally going to be worse than getting a molotov cocktail thrown at you, but not worse than getting napalm dropped on you.
So what is the BENEFIT of not taking magic?
I didn't say there is a benefit
The highest AC builds are usually magic using characters, like Bladesingers, forge clerics, and Eldritch Knights (which I know is a fighter subclass, but they are the fighter subclass that USES MAGIC, so what is the benefit of forgoing that?)
If you want a non-magic character, the benefit is having a non-magic character (and a lower AC). Just like if you want a character with good social skills you will have less hit points.
Damage? I'll admit, Fighters and Rogues are not slouches when it comes to single target damage, but generally, full caster optimizers SCOFF at the idea of building blasters. It is the LEAST effective thing you can do, in their mind. And trust me, I've seen first hand it doesn't matter how much damage you can do if the enemy can't touch you for 10 turns of combat.
It depends on the enemy and at higher levels as control spells become more powerful and available they also become less reliable as save bonuses or flat condition resistances outstrip the increase in DC and legendary resistance becomes available when they do fail.
Sure, Speak with Dead is a great spell for solving murder mysteries. It is also not even a blip on the design space of full casters.
But it is an example you used and it comes with a high cost of being prepared and using a slot. The real value in the investigator being able to get clues (albeit less effectively) is letting the Wizard prepare Fear instead.
It is going to be rare that a character is walking around with Speak with Dead prepared.
What does the 11th level fighter do that is impossible? Make more attacks? Re-roll a save? That's it. And those aren't impossible things. Those aren't even notable things, in terms of plot impact. No challenge or encounter is going to be circumvented or even significantly impacted by that.
An 11th level fighter with a 6 constitution can fall off a 70 foot building and survive 100% of the time and even walk away with no broken bones. They can fall off a 200 foot building and survive over 60% of the time.
On top every fighter subclass except Samauri, Champion and Cavalier get an explicitly impossible ability by level 7.