Sanji and Zoro have in universe absolute faith in him.
Current Luffy is matching hybrid Kaido.
"the plot demanded":
that Luffy not use FS against Kaido pre-Udon
that his scales had a magical property that only ryuo could overcome,
that Rayleigh didn't bother to teach Luffy barrier ryuo despite it barrier being portrayed as basic against the elephant and used by much weaker characters.
that Luffy runs around rather than rocketing up to the roof to fight with the scabbards,
and that Law decides to fuck around with the poneglyph as Kiku loses her arm and the scabbards "almost die"
that Luffy burn completely through his g4 stamina, despite Kaido showing fear from internal destruction red hawk.
that Luffy can only beat Kaido after his subordinates have finished their fights.
That last one is the key.