well at least it has nothing to do with the quality of the writing, because the current writing of one piece is mediocre at best.
luffy is expected to get a lot of special treatment, especially in the new world.
"plot armor, asspuls, deus ex machina, plot convenience" these are essential parts of luffy's character.
luffy is a character who: gets a lot of help, unlimited chances and power ups, and suffer no consequences. he is above the rules.
here's some scenarios that would be out of character for luffy:
-luffy wins without help
-luffy gets only one power up in a fight
-luffy suffer long term plot altering consequences
no one can complain about how luffy gets to fight multiple rounds against kaido, or how he got the power of telepathy, or how caribou was conveniently hanging out at tokage port carrying large amounts of food, or how shinobu can make people older instead of rotting them, or how momonosuke flew luffy to onigashima before he learned how eastern dragon fly, or how luffy's devil fruit got retconned from paramecia to mythical zoan...
when it comes to luffy people suspend their disbelieve for the sake of hype, but they never keep the same energy with the other characters.
non-MC characters:
they are not allowed to receive any kind of assistance
they are only granted a single power up
they don't get second chances.
that is why people react negatively when a non-MC character receives MC treatment:
-nami got zeus permanently fused with her climate tact despite big mom taking away his soul
-zoro gets temporarily healed with an asspull mink medicine
-law and kid asspulled awakening
-zoro tames enma and gets AdvCoC at the same time
-sanji conveniently awakens his genes at the right moment and gets increases strength, durability, regenerations at the same time
also i don't remember people raging over chopper getting monster point duration boost, but that's mainly because no one give a shit about chopper.