Well if you want a hypothetical scenario, imagine if Sanji bumps into Caribou attacking a bunch of mermaids. Caribou's not that strong in general compared to Sanji, but he's got a Logia fruit… so, the Black Leg's utterly useless? I just don't like the sound of that. Now I read the theories about heat and mud and whatever, but I feel as if Haki is a way for Oda to get around that problem, to not have to have everything conveniently line up so that whoever's fighting a Logia user somehow happens to be sitting on their natural weakness. Or let's say that someone like Enel shows up, and Luffy is inconveniently absent for whatever reason. The whole crew gets slaughtered just like before? I dunno, I can't help but feel that there'd be something wrong with that, especially with the rather heavy theme of "Luffy can't do everything on his own", and the timeskip was a great opportunity to remedy a problem like that given that the means of getting around it - Haki - takes a lot of time to be able to even use at a basic level.
haki would be nice, but the fire and mud therory works. same with luffy vs enel scenario. haki isn't easy to learn, and not only that, sanji was never there to learn haki. he was learning how to cook + how to escape + defeat the 99 masters, but he nvr had actual knowledge of haki. i doubt he has haki, but doesn't mean he doest' have any way of dealing with logia