Despite some decisions that I would have made differently from Oda, overall I liked WCI and Sanji's participation. I just needed some nice 1v1 to make it perfect. But I totally understand those who didn't like it.
As I said, it was very splitting.
Imo WCI was to showcase how important Sanji is to Luffy (like a literal left hand) and everyone else like Nami (they got some moments tho), Chopper and even Jinbei, even as a new crewmate, and the things he is good beside fighting. Sanji didn't showed any weakness on fighting during the whole arc (aside from the Raid Suit showing very useful, foreshadowing that he would get one), considering he was busy 99% of the time while enemies like Oven and Daifuke were chasing him like crazy at once, he could keep up against them with ease, Oda just hold him to grow along with Zoro now at Wano, while Luffy needed to develop a lot cuz he would need to grow a lot compared to Zoro/Sanji anyway (and thats proven to be right with the latest chapters).
Imagine if Sanji fought a Sweet Commander? People would shit on Zoro, and Sanji would look too much ahead and his bounty would skyrocket, i don't think it was Oda's purpose to do this to be fair, especially storywise