You didn't argue this very well, but I agree. The problem isn't that there isn't market space for other types of comics, it's that the money in the industry never succeeds in pushing them. There's nothing about comics as a medium which makes them inherently niche; newspaper comics were huge for years, and Japan/Korea/Europe have tons of very popular series for adults. They even work well on phones, and could easily adapt to digital distribution– if Netflix can toss hundreds of millions of dollars at original shows, then a comics streaming service can toss a requisite amount at a book lineup.
Comics will never be more than a weird, childish level of consoomerist nerdom unless something shakes the industry up enough to bring it closer to manga in terms of tone, scope, style. You can find manga about dropout failures going to agricultural school, making friends, learning about the meaning of adulthood. Anything like that made in the west will be published one page every six years by a retarded self-obsessed webcomic eceleb. The medium is sick in the west, it's unwell.