Ovinomancer
No flips for you!
Per the OP, the reason you might consider having guns do more damage is that they are flat out more lethal than any muscle powered weapon you'd like to consider. The examples of the lethality of swords in the OP, for instance, rely on the target being entirely passive so that the attacker can wind up their attack perfectly. This wind up, though, would leave an actual combatant in a fight vulnerable. Like a haymaker punch, if it lands, it's bad, but it's hard to land such a blow against a ready and reasonably capable foe. The counterpoint to the sword attacks claimed in the OP is putting the muzzle of the gun to the base of the skull behind the ear, with a slight upward angle, and pulling the trigger. The damage in perfect, non-combat situation is roughly equivalent -- the target dies.
In combat, though, where you're attacking a person that's trying to not get hit and do you harm, you're much more likely to survive a sword or knife blow than a gunshot. There's a lot of evidence for this, in that even in the rarer attacks with knives or swords, people rarely die, whereas guns routinely kill at a much higher percentage. If you assume that you're facing an opponent that is actively trying to not get hit, but you have landed a blow anyway, the odds that the blow is lethal with a sword is much lower than the odds of a lethal blow with a firearm, even a flintlock one.
Does this mean you need to make firearms do more damage in D&D? No. Of course not. It's a game, and you should choose an option that makes the game best for your table. However, citing anything real world for the argument that swords do as much damage as a gun is silly -- there's tons of real world examples that showcase the opposite. Heck, the normal result for being shot in a limb with a flintlock, if your survived, was to lose the limb. This isn't the case with a sword, where that was a possibility, yes, but not the normal expectation. The musketballs traveled at enough speed that impact and subsequent hydrostatic shock shattered bones and liquified muscles, connective tissues, and blood vessels. Sword might sever muscle and break bones, yes, but angle of attack could very easily protect arteries, so a blow from a sword to the arm would be unlikely to cause arterial damage. This is critically important because you can heal a broken arm and severed muscles with bloodflow to the area. It wasn't until modern medicine and the ability to repair damaged arteries that gunshots became less lethal, and we live in that world. It's hard to intuitively grasp what combat with firearms was actually like when the opponent was likely to be armed with a sword -- it was horrible and brutal and getting shot was maiming if not lethal. And all of that makes for a very specific kind of game appeal, so it's usually elided and you get a d8 for a flintlock pistol in 5e.
In combat, though, where you're attacking a person that's trying to not get hit and do you harm, you're much more likely to survive a sword or knife blow than a gunshot. There's a lot of evidence for this, in that even in the rarer attacks with knives or swords, people rarely die, whereas guns routinely kill at a much higher percentage. If you assume that you're facing an opponent that is actively trying to not get hit, but you have landed a blow anyway, the odds that the blow is lethal with a sword is much lower than the odds of a lethal blow with a firearm, even a flintlock one.
Does this mean you need to make firearms do more damage in D&D? No. Of course not. It's a game, and you should choose an option that makes the game best for your table. However, citing anything real world for the argument that swords do as much damage as a gun is silly -- there's tons of real world examples that showcase the opposite. Heck, the normal result for being shot in a limb with a flintlock, if your survived, was to lose the limb. This isn't the case with a sword, where that was a possibility, yes, but not the normal expectation. The musketballs traveled at enough speed that impact and subsequent hydrostatic shock shattered bones and liquified muscles, connective tissues, and blood vessels. Sword might sever muscle and break bones, yes, but angle of attack could very easily protect arteries, so a blow from a sword to the arm would be unlikely to cause arterial damage. This is critically important because you can heal a broken arm and severed muscles with bloodflow to the area. It wasn't until modern medicine and the ability to repair damaged arteries that gunshots became less lethal, and we live in that world. It's hard to intuitively grasp what combat with firearms was actually like when the opponent was likely to be armed with a sword -- it was horrible and brutal and getting shot was maiming if not lethal. And all of that makes for a very specific kind of game appeal, so it's usually elided and you get a d8 for a flintlock pistol in 5e.