its impossible for sanji to win. doflamingo is on par with law.
the moment doflaming tanked sanji's diable jambe the fight was over.
but if sanji wins that would put the monster trio only below admirals and yonkou...and mihawk.
Dom isn't just on par with Law he's above Law. Law obviously doesn't want to fight Don 1 on 1. I know Funitora got in the way this time, but Law made sure to dodge Don at Punk Hazzard. If Sanji beat Don (which isn't happening) Zoro and Luffy would probably be at Admiral level given that they are stronger than Sanji. They will reach and probably exceed these levels eventually, but it's half a decade too early. But seriously, if Sanji beat Don, Zoro could probably beat Funitora and then Luffy could probably challenge Kaido with some assistance.
I think Law said something about draging Don from Fujitora so he can fight him alone on the bridge. It would be like that if Don didn't saw Strawhat ship and decided to attack them first.
It would be a nice change of pace to have someone other than Luffy defeat the main antagonist of an arc IMO, but I don't think Sanji stands a chance to be honest Doflamingo seems too close to Admiral level for him to go down, I don't think the straw hats can tackle with them individually quite yet, I'd say maybe the monster trio or maybe the entire crew could match an admiral and even that might be pushing it