I think for Sanji to complete the Mythical Zoan of Luffy and Zoros 2 meito and one cursed blade, it is only fair for him to have something like exoskeleton, now let me explain.
Lets compare to Zoros swords and Sanjis exoskeleton first. Enma which is one of the great 21 swords and it has Odens soul, lets also don't forget Enma lets Zoro to use immense amounts of haki compared to Luffy's gear 4. Of course that amount of of haki usage can kill Zoro but it also lets him grow stronger for a limited amount of time. Wado ichimonji is also a sacred blade that is compared with Enma it is in the same legue with is without cused blade mechanism.
Wado was crafted by Kozaburo for Kuina so we can't expect from it to fall behind Enma which was also crafted by Kozaburo. Lets also not forget Wado didn't break against Mihawks attack and let me say this if Mihawk did the same attack to Sanjis exoskeleton he would have cut him undoubtedly unlike Queen who broke his cheap sword.
Sandai Kitetsu though it may be weak compared to others it a cursed blade that is crafted by Wonos shogun who can craft swords as good as Enma or Wado. Sandai may also be the sharpest sword Zoro has.
Now lets come to Luffys mythical zon fruit, in the past people always said Sanji is weak because Luffy only has gomugomu nomi but it turned out it was a deity DF that fought with the worlds current ruler. Sanji always needed to do better cause gomu was just a weak paramecia but it turned out it wasn't.
Luffy DF gives him ablity to stretch and bend reality also gigantification, with density of rubber like body Luffy can hit steel like punches when he uses armament hardening. Lets also not mention superhuman speed when he used gear 2 which he seems to be able to use easily now with his bodys flexibility.
Luffys Nika awakening also gives him looney toons like cartoonish powers and even Kaidous mythical Dragon DF can't beat. Though Kaido wasn't awakened he was much older and experienced haki user.
Luffys DF also lets him tank immense amount of damage and taking punishment.
Anyway what i wanted to say is even with exoskeleton Sanji is still the underdog.
Lets compare to Zoros swords and Sanjis exoskeleton first. Enma which is one of the great 21 swords and it has Odens soul, lets also don't forget Enma lets Zoro to use immense amounts of haki compared to Luffy's gear 4. Of course that amount of of haki usage can kill Zoro but it also lets him grow stronger for a limited amount of time. Wado ichimonji is also a sacred blade that is compared with Enma it is in the same legue with is without cused blade mechanism.
Wado was crafted by Kozaburo for Kuina so we can't expect from it to fall behind Enma which was also crafted by Kozaburo. Lets also not forget Wado didn't break against Mihawks attack and let me say this if Mihawk did the same attack to Sanjis exoskeleton he would have cut him undoubtedly unlike Queen who broke his cheap sword.
Sandai Kitetsu though it may be weak compared to others it a cursed blade that is crafted by Wonos shogun who can craft swords as good as Enma or Wado. Sandai may also be the sharpest sword Zoro has.
Now lets come to Luffys mythical zon fruit, in the past people always said Sanji is weak because Luffy only has gomugomu nomi but it turned out it was a deity DF that fought with the worlds current ruler. Sanji always needed to do better cause gomu was just a weak paramecia but it turned out it wasn't.
Luffy DF gives him ablity to stretch and bend reality also gigantification, with density of rubber like body Luffy can hit steel like punches when he uses armament hardening. Lets also not mention superhuman speed when he used gear 2 which he seems to be able to use easily now with his bodys flexibility.
Luffys Nika awakening also gives him looney toons like cartoonish powers and even Kaidous mythical Dragon DF can't beat. Though Kaido wasn't awakened he was much older and experienced haki user.
Luffys DF also lets him tank immense amount of damage and taking punishment.
Anyway what i wanted to say is even with exoskeleton Sanji is still the underdog.