It's not about character development but rather Luffy's personal worth, it's about perfect balance between Luffy achieving something because ha has faith in his frends and Luffy achieving something because his friends have faith in Luffy. Luffy always relies in his frends in every situation and there's been moments when Luffy got helped in fighting by his friends, but there are moments when his frends look up to him as a captain as a someone who is responsible for their lifes and needs to have the strength to protect them, it's about Luffy having what it takes to become the Pirate King vs Luffy becoming the Pirate King because he was made the one by his friends. Like when Luffy showed his frends especially Robin that he can defeat Rob Lucci and protect them, protect Robin...
Or when Nami relied on Luffy when they couldn't do anything...
And Luffy humiliated himself in WCI running form Big Mom cuz he didn't have the strength to defeat her, then he relied on Jinbe for covering their back while they were running, Pedro died because Luffy couldn't protect him.
And now, they are in Wano and their's no way to run, Sanji put his faith on Luffy when he dicided to go with them and Luffy should prove himself, the Wano people relie on Luffy, Momonoske relies on Luffy cuz he promised Momonoske that he will defeat Kiado.
The Samurais needs a real man to fallow, someone that can defeat Kaido...
Luffy alredy relied on Jinbe in WCI and now his friends relie on Luffy, if Luffy can't defeat Kaido by himself, prove his personal worth, back up his words, protect his frends, then what kind of Pirate king he is?