Hmm. A couple of things.
The 99 Carnwenhan is an incredible weapon. Even without being upgraded to 119 it's an incredible weapon. I don't think you'll be disappointed with the 99 unless you made it as a DD weapon.
It stacks with pretty much everything. Gjallarhorn, Daurdabla, Soul Voice, Clarion Call, Marcato, Troubadour and equipment. Bard's best weapon is 119 Carnwenhan. Bard's second best weapon is 99 Carnwenhan. Other jobs 2nd best weapons aren't their 99 relic/mythic/empyrean for the majority.
I bet RDMs wished their mythic gave Enfeebling magic duration+50%/Enhancing Magic duration+50%, and upgradable with 228 magic accuracy skill on top of it. And then we have troubadour as well to double it. Or other jobs wishing their relics, mythic and empyrean all stacked together.
Right now for debuffs I use either Carnwenhan for duration or Izhiikoh for magic accuracy skill.
Carnwenhan99
Duration+50%
Magic Accuracy+25
Izhiikoh
Magic accuracy skill +188
The upgraded 119 Carnwenhan has
Duration+50%
Magic accuracy+25
Magic accuracy skill +228
So it would save me an inventory spot and gives more magic accuracy skill and gives the duration too. Carnwenhan's 2nd best use is for long duration lullabies so the magic accuracy skill is welcome. Plus we just got lullaby+1 hands and song duration feet.
To me it's a worthwhile upgrade. Whether it is to others probably depends on how much gil they have and what other weapons they own. If I had a DD mythic/relic/empyrean I would probably upgrade that first since the 99 Carnwenhan is so good. We are probably lucky we have the luxury of choosing whether it's worth it or not. Some of the other weapons are either unusable without being upgraded, worse than their delve boss counterpart when upgraded or only slightly better despite the the increase in difficulty of obtaining.

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