Regarding the OP -- On a basic level, most of what Easton says about broad/back/arming swords over rapiers holds true. It is an interesting, thoughtful analysis. However, as counterpoint, most of the negative things this video states about rapiers can also be assigned to spears. There are pros and cons to each weapon (they both stuck around for incredibly long times, after all).
As to what would be best for D&D, honestly it really depends on how you envision your character's fights (how often do your characters fight in military formation? face off against multiple foes at a time in general?). Similarly, and as others have alluded to, real world weapons were not built with going down into conveniently placed holes in the ground to face dragons and ghouls, and if we were to go completely 'realistic' to that scenario, a whole new set of weapons would have been developed in a world populated by such challenges (and those would be the real optimal choices).
But at the end of the day, I think Umbran has the right of this, it is cherry picking. Not that that is inherently bad (after all, what one needs for a game world to have verisimilitude is an inherently subjective judgement call), just to be judicious with it. There's that adage about "if I am a pedant, someone else is 'ignorant' if they don't know what I just learned" and we in nerd culture do love our trivia. I certainly remember suddenly getting very invested in the realism of some facet of the game after I learned a new fact about medieval/renaissance arms&armor (or anything else).
The question I would pose is: does doing anything to reflect this newfound knowledge actually benefit the game to be included? Likewise, do the game rules reflect equal distinctions which reflect the benefits/disadvantages that alternative options have?
As if I needed more reason to hate them.
Rapiers and bards go together like vaping and that guy in the fedora who wants to tell you about NFTs.
Bards were still bards when back in AD&D when they were using longswords like everyone else.
I'm familiar with this particular channel; it makes the rounds among HEMA & martial reconstructionist devotees-- one really has to take their opinions on fuzzy-and-not-very-clear medieval & renaissance manuscripts with a grain of salt.
As alluded briefly by Ungainlytitan above, the rapier evolved out of very specific circumstances; it was not strictly speaking a "military" weapon like a broadsword was. For the time period and purposes it was used, it did its job quite well.
Easton has been pretty upfront about the virtues of the rapiers as well. The advantage of the reach is phenomenal, they do not shatter with every slight sneeze as they are sometimes depicted, they can cut, and they even were at times used on the battlefield. The thing I've mostly gleaned from his and other similar channels is that for every point there is a counterpoint; for every rule an exception; and for every known there is an unknown, contested point, or guesswork. Nothing is pure, the sources conflict, and we don't know how much what has survived is representative.
So, when do we start asking about the environment in the dungeons and how things live down there?
They have meal plans at the dungeon cafeteria. Otherwise, their employee badges work as stored-value cards at the food court.
I don’t think rapiers were in D&D originally if my memory serves. They were added in 3e I think, possibly due to the popularity of Princess Bride!
Original D&D had 'Sword' and 'Two Handed Sword.'
Supplement III: Eldritch Wizardry distinguished that some swords were sickle-shaped (since druids could use those), but no rules on what proportion. The swords got split out for AD&D with various types each with different stats and the miss-attribution of what was a longsword and so on. 2E expanded the list, and rapier showed up as a distinct entry in AD&D 2nd Edition's
Complete Fighter's Guide (along with the swashbuckler kit/character option).
Imputing reasoning to the TSR era is always a hazardous activity, but it coming along with the swashbuckler kit (and alongside cutlass and pirate kit, katana&wakazashi and samurai kit, etc.) make me think they simply were making the game work better for depictions of characters.
Chainmail was a historic 'army-men' game with a fantasy subsection, and as such was very battlefield-based (ex.: oD&D and basic-classic have magic users use daggers because quarterstaves didn't show up in Chainmail weapons charts). The game lurched along with additions like staves and clubs (and a whole new list for Oriental Adventures, which is a thread by itself), and then with 2e they realized they could make a lot of content by helping people play Aramis and Captain Hook and Captain Caveman and pajama ninjas and so on.
It worked for Robin Hood. D&D is about tropes, not logic.
There's certainly a lot there. Same as how castle courtyards are still open-air in a game with griffons and dragons. Some things exist in the game because we want to play our favorite 'long-ago, but with magic and dungeons' scenarios, not what it would actually have looked like long ago if there was magic and dungeons.
*I call it this because for me, the tipping point was when I saw an article about how Isaac Asimov (of Laws of Robotics and Golden Age of Science Fiction fame) was a serial and well-known sexual harasser. I mean ... this was the guy that was famous for not drinking and not including any sex and barely a hint of romance in his writing, which he churned out so prolifically that even Stephen King thought he should slow down. Ugh.
In his novels. Perhaps you aren't aware of:
His 80s&90s books also had sex, IIRC.
But, back to the OP.
Rapiers are good for adventurers, because they are good for players. Because they reveal the others at the table who cherrypick their realism with insufficient consideration of anyone else's fun but their own.
I mean, that depends on who does what. If yeah you suddenly decide you must harp on the realism of the game when everyone else is just trying to play it, that's not great. On the other hand, if you are designing a game (or just a shared consensus on what type of game you want to play), than more power to you.
Okay, fair point. I should have said a spear, as long you can make it without jamming up at every corner.
The main point was that swords are designed for fighting humans, not monsters, and the longer blades are specialized for particular forms of human-fighting. There's a reason nobody hunted animals with swords. A rapier in the dungeon is unrealistic, but no more so than a greatsword.
I think for all the heavily armored giant monsters, perhaps something halfway between a can opener and specialized crab/lobster dining utensils might be in order.