You're alluding to something that is a numbers issue. Which I agree that DPS output would have to be tweaked in some way. The focus of my criticism is the gameplay.
Dualcast was added because FFV's developers needed a gimmick for RDM to fit into that game's job system. At the baseline the job had nothing to offer in terms of learnable abilities that you could cross class, so Sakaguchi & Co. had to make something up. Dualcast as a small part of the greater whole of RDM doesn't bother me (which is why even my redesign of the job has it), but it's pretty clear FFXIV's devs wanted it to be the focus of the job's gameplay, to the point the job's systems were all built around it.And while Dualcast is added in FFV that's only two games previously with Red Mages, not exactly a large number. And Dualcast helped solidify Red Mages as being speedy casters, along with the weakness in terms of spell strength. So it's very much a defining part of the job.
If we want to go into detail, the main adjustments needed for the Sage design to be complete would be changing the way the spenders work (I'm partial to reaching 80/80, casting not-Flare to spend the black mana bar, casting not-Holy to spend the white mana bar, then casting not-Meteor as the big finish), and changing Dualcast into a mechanic that reduces cast time as you continuously cast spells (stacks 4 or 5 times, reducing casting time by up to 40-50%).As for Sage versus Red Mage, the two are very similar in a lot of ways so it's hardly surprising that you could rework Red Mage to be like Sage. Though even then, you're talking about removing eight different moves which is a decent amount, and Sages are more known for being... well, slow and heavy spellcasters, not quick and speedy like Red Mages. So you'd have to make more changes than just chucking the sword skills.
My point in all this is that sword use is very disconnected, to the point of being almost token. I've never seen that as a good thing.
The way I see it is that you can have RDM without Dualcast, but you can't have RDM without the sword. The sword has been part of its aesthetic since the start of the series, and with that came the idea that RDM is hardier than your average caster in combat and could pull their weight in a melee. No one would expect RDM swordplay to be doing fighter/warrior-level DPS, but would expect them to supplement said swordplay with magic.In terms of Red Mage throughout the series, I'm not sure how important the swordplay element really wound up being. It's an aspect, but most of their gimmick has been speedy spellcasting I would argue.
The enchanted sword thing is fairly recent. It was one of the good evolutions of concept seen in FFXI. The difference is that FFXI's RDM could cast Enfire on themselves and whack away at a mob while also being able to cast spells as needed. FFXIV's RDM has to get permission from a mana bar before their sword swings are worthwhile.The sword is leaning more towards being enchanted (which is what we already have here), and is mostly a means to round out their kit a bit.