For all those asking how sanji would have dealt with fire, the raid suit is actually resistant to fire if you remember. Sanji literally lights his legs on fire all the time and feels no pain. Fire based attacks would be the least of his problems.
And in response to the thread, there's no way he would have done as significant damage to kaido as zoro could considering he doesn't have swords or Enma nor is he as strong as zoro.
However he would have been useful in other ways such as providing support and doing the occasional poke damage to kaido and distracting him with his invisibility so the big guns like Luffy, Law and even Kidd can land heavy blows.
In addition, I really doubt Zoro is the only supernova who can block hakai let's be realistic here. Luffy has shown proficient enough haki to deal with it in the same manner Zoro did, Kidd could have surrounded himself with a lot of metal and let the metal take most of the force, Law could have used his curtain ability or some other sort of hax he could conjure up to stop it for a minute.
If Sanji was faced with hakai, assuming he tried to block it with his cape or something he would have taken worse damage than Zoro for sure but he isn't dying don't delude yourselves.