There's few people who upload their scanlations originally to Mangadex (and normally they do it both on here and e-hentai/ex-hentai. So the majority of uploads to Mangadex are requests fulfilled by power uploaders. And about the amount of new hentai, if it serves as reference, I stopped like last year cheking new works because the quality and variety dropped like a nuke and even the authors I liked became boring. There are certain factors to consider:
1st - Hentai manga magazines are dropping like flies, since authors that get popular prefer to self-publish and new authors or residual authors lack in originality.
2nd - Unless they sell well, they prefer to adapt to SFW manga (including borderline manga) to avoid taking loses and aspire to serialisations.
3rd - Fanbox subscriptions keep hurting with a higher paywall and in the end they don't publish full stories but pages in single digits.
4rd - Forget about story, specially with the rise of AI generated content is straight up porn with comissions and NTF involved.
So, in short, better leaving the ship before it sinks.