Chaosmancer
Legend
Dual wielder isn't needed with two light weapons. It's needed to use the rapier as part of the combo. As long as both weapons are light the first attack succeeds with a vex weapon for advantage the second weapon is set up for a sneak attack. The nick property on the second weapon keeps the bonus action available. 2014 dual wielder doesn't keep the bonus action because there's a lack of the nick property.
Dual wielder used to even up an odd stat does make this work with a rapier and scimitar because the scimitar is light. Setting up our own sneak attack through dual wielding like that seems like a nice bonus when we can also keep cunning action, especially since cunning strike relies on that sneak attack condition. It creates an option to sneak attack without having an ally nearby.
That plus something like fast hands (that can now use a magic item in the options) allows me to dual wield effectively on a thief and not give up those bonus action options. Vex attack, nick sneak attack, apply an appropriate cunning strike option, use fast hands as desired.
That particular combo is less useful on a swashbuckler, but it keeps cunning action available for rogue subclasses. The opportunity cost between cunning action and twf is gone using the combo. It looks like a solid upgrade to the class to me unless I'm missing something.
You seem to be talking about something COMPLETELY different than me.
You said that the rapier not being light hurts it for Dual-wielding. But in 2014 the Rapier was never a light weapon. It never could be used for dual-wielding without the feat. That was always a necessary step.
You then went on to discuss shortsword and dagger, which is 1d6 and 1d4... but the rapier also has Vex, so if you have the feat, which you must to dual-wield rapiers at all, you would use Rapier and Scimitar... which is basically identical and does 1d8 and 1d6.
I know you mentioned throwing daggers, but since you can sheath a weapon after your attack and draw a new weapon before your attack... Rapier Scimitar wouldn't prevent throwing daggers, and the feat allows you to draw two weapons instead of one.
The only real difference I'm seeing is that you can no longer dual-wield rapiers. But, even if you are just talking "I don't need the feat to dual-wield" this has always been true. Dual Shortsword was 2d6 compared to dual rapier being 2d8. So I'm very confused why you think Dual-Wielding somehow obviates the Rapier, when it works exactly like it used to work... just now with identical weapon masteries.