I don't know. Instead I'll focus on my subjective experience with comics and manga, as a nobody from LatAm who likes fantasy.
Manga is something that I grew up with. As adulthood came by, I didn't feel the need to ditch it - instead I found other manga series to enjoy. There's stuff for young kids and adults; spicy and tame; comedic and serious; romance and no romance. No matter who you are and the stuff that you like, I feel like you could find at least one enjoyable manga series to read.
In the meantime, what I've found from comics elsewhere:
- Local (at least in Brazil) - either tailored for kids (see: Monica's Gang) or newspaper 4-koma with social commentary (see: anything from Glauco). So only kids get actual stories? Based on Mafalda I feel like that's how the cookie crumbles in Latin America as a whole.
- European - wider in age demographic than the local ones, and some do have fantasy (Even erotica. Druuna, I'm looking at you. ~~And your butt.~~), but I feel like they lack dynamic. Even adventure ones like Tintin. Still enjoyable to read, but sometimes my cup of tea might be yerba or coffee, you know?
- United-Statian - Mary Sue protag got superpowers from Z'bh'thy, and now is fighting the Evil for the sake of their country. Skip past 20 years and they're still in the same slop, never reaching the end, in a multiverse that makes my PC cabling look tidy in comparison.
- manhua (China) - I actually found quite a few enjoyable series (like the Fairy Captivity, Yaoguai Mingdan, My Wife is the Demon Queen). Perhaps not surprisingly they're similar in spirit to Japanese manga. I could see myself reading more of that stuff. (I'll skip wuxia though.)
- manhwa (S. Korea) - 90% of the stuff that I've seen boils down to either "adultery stories" (I'm not into that stuff) or what feels like ultra-shōnen: "level ZZ is not enough, MC needs to reach level ZZZ". That said I did find a few enjoyable series, like FFF-Trashero or Carnivorous Princess Yegrinna.
Are they always like this? Probably not; I bet that people can find exceptions to every single bullet point that I've listed.
Something must be also said about the synergy between light novels, manga, and anime: if you want you get to enjoy the same story thrice, in three different media, and the pleasure associated with each will be different. And if the story is good enough it won't tire you down. I simply don't feel the same in non-Japanese series, even the ones that adapt the same universe across different media (like X-Men).