The longsword does not fit the 5e definition of finesse better than the Rapier IMO. Neither in execution nor in the physical properties of the weapon. It's a heavier weapon with a larger range of movement than the Rapier, especially wielded in one hand (although actual Medieval/Renaissance Longswords were primarily two handed weapons). I'm referring mostly to actual Medieval Longsword technique for the range of motions, since D&D doesn't get into that at all. I have fought with both weapons, and while this is entirely anecdotal, I personally found the longsword to be enormously more tiring and reliant on muscle to keep fighting with. That's not proof of anything, but it is my personal experience, and it fits with the evidence I have for how longswords were used. That's not to say that longsword combat doesn't also require speed and dexterity, because of course it does.
What we're running into here is D&D's rather artificial split of DEX and STR. Most combat techniques are very much a combination of the two, and not strictly one or the other.
What we're running into here is D&D's rather artificial split of DEX and STR. Most combat techniques are very much a combination of the two, and not strictly one or the other.