@inKraken Same. I've bought many manga series I discovered through this site. Like hundreds of issues. I signed up with Amazon.co.jp to purchase them.
Whole series like Alfheim no Kishi and Nekomix Toraji and Inuyasha... (Practically everything Lovely Strange Dark scanlates, I've bought.) Especially for old series without official English release, like most of the old shoujo series- I download English scans, then buy. (Some old stuff like Red River and Basara have an official English release, so I bought them on American Kindle instead.)
Manga-ka have earned hundreds of dollars off me using this site. Most of these series I never would have heard of without mangadex, so it's not like they're losing any money from it. Granted I prefer older stuff. It may be different for whatever the popular series are these days. (I do buy Yona and Red Haired Snow White and Conan tho.)
Even a good chunk of official releases are due to scans: I don't think they would have brought over Vengeful White Cat manga and novels without me and lots of others begging. (I bought the Japanese releases, and now I buy the English ones also.) A lot of the demand for series outside the usual Shounen megahits is from fans who read scans and request official releases.