I think the grading is based more on the craftmanship level of the blade. How fine the blade and when will it break. Sandai and Yubashiri are a good example in this regard. And Kaidos weapon would be legendary because it was wielded by Kaido and it did not break even though it was ungraded.
Heavily disagree. At this point, the grading is based on the potential and strength of the blade from the way Oda is grading swords. He's not making any distinction
Only top tiers have wielded Supreme grades, they are a symbol of power more than anything else
Yoru is called the "strongest" sword in the world not the "most durable" sword in the world
Enma is Great Grade cuz it has a property of using haki to reinforce itself. Kozaburo clearly spoke of it thru its cursed strength not its durability
And then you can increase the rank of a sword by turning it black, which is obv meant to be a very special feat for CoA
Kaido kanabo is legendary cuz it was wielded by him, a top tier. Oda spoke nothing about its durability or not breaking while being used by Kaido
Fujitora's cane stick turned out to be a Saijo cuz he's simply an ADMIRAL and a Great Grade would be disgraceful on him
So yes I believe swords like Vohu Bohu Gram Longinus will remain ungraded, while Muramasa can be graded as an original Saijo that was lost to existence or some shit. That will put them in the same class anyways cuz of their status as OG 19 swords while Muramasa association with Saijos, but at the same time not be capped by the portrayal of one of the 12 Saijo "beneath Yoru" (since they are def not found in the book of swords that Tashigi had)
It's similar to why Kikoku is ungraded, Oda telling us Law is growing with his sword and will eventually reach some level, maybe 12 Saijo, maybe further. It is painfully clear based on the way Oda keeps shit vague to maintain some semblance of hype while saving face for other things