It is very easy to see how using one-shot muzzle-loaders would leave the sword (and other blade weapons) as major weapons in warfare, as this is how it was historically for quite some time.
One point to clarify is that for a LOT of fighting men in history, the "sword" was rarely the PRIMARY weapon used for killing opponents on the battlefield. Yes, yes, yes ... Gladius Hispania+Roman Legions ... for a while the sword WAS something of a primary infantry fighting weapon, but that's more of an exception than the rule. In our modern times, we've romanticized the SWORD so much as to have this peculiar idea that it was the Best Main Killing Weapon of all time in all fighting eras.
Unfortunately, that's not the case.
As it turns out, the SPEAR is the main battlefield melee weapon for the majority of world history, with the bow/crossbow being the main battlefield ranged weapon for the majority of world history.
The sword, was usually a BACKUP weapon(!) ... even in the age of knights and chivalry.
Additionally, swords are really good against opponents who lack body ARMOR ... but against armored opponents, swords are actually NOT your best option. For "stabbing into gaps and weak points" you're actually better off with a dagger (which is easier to control the point on), which is why half-swording is a technique to use against armored opponents.
Against ARMORED opponents, you're better off going with blunt weapons (maces, hammers, etc.) which will transfer the shock force of impact THROUGH the armor and into the flesh and (skeletal) structure of your target, potentially breaking bones, in addition to bashing up the armor (which if it's rigid metal can ruin the fit and articulation, reducing freedom of movement, which is BAD™).
So swords are good against "soft" (fleshy) targets, but not so good against armor (particularly metal armors like mail, brigandine and plate).
On top of all of that, there's the fact that human bodies are surprisingly capable of surviving lacerations (even amputations!) caused by sword strikes. Getting cut with a sword can be damaging, but it probably won't kill you (aside from the more obvious devastatingly damaging cuts, of course). Piercing damage though, will often times be more disabling and ultimately lethal. Poke a hole in someone and they're more likely to die from that hole put into them than if you slashed them for the same amount of "damage" (in the RPG sense).
Anyone who would like to know more about the history of weapons in warfare, I'll refer you to "Captain Context" Matt Easton and his
scholagladiatoria youtube channel, where he talks about all kinds of weapons and how they were used historically.
