So, this is something Greg of the One Piece Podcast speculated about that I found interesting, but he speculated that a major factor to the success of Kimetsu post-anime as well as the like of MHA is that protagonists who are built on ideas like compassion and have large amounts of internal thought appeal far more to modern readers than hot-blooded idiot heroes like Luffy. Greg's an elementary school teacher if I recall, so he does know what he's talking about.
So, what happened? Anyone has an idea as to why the stereotype hero that is associated with the likes of Goku/Luffy/Naruto/Gon/Asta has gone out of favour?
Eh. This is just a random statement.
Most series that have a calm MC usually has a fiery side kick and an edgy/cool character to compensate for it. Usually the latter two characters end up being more popular than the actual MC.
Like, Bakugou literally was voted the most popular character in the latest MHA poll. Previous poll had Bakugou and Todoroki ahead of Deku.
Whereas, for One Piece - Luffy has been the consistent favorite character in all polls.
So this argument that hot-blooded heroes are out of favor is just a misplaced opinion, at best.
UxU literally has a hot blooded MC.
Side Note - Gon is far from your stereotypical battle manga MC. He is not hot blooded at all. He's passionate in many ways in his actions but he doesn't rush into a fight without thinking. Two very different things. Deku has clear inspirations from Gon, if anything. The times Gon became hot blooded, they became pivotal turning points in HxH's story because of the consequences of his actions.
Also, unlike most other MCs, Gon's age shines through. He doesn't act like an adult unlike most other MCs. His innocence as a kid is what makes him so lovable.