"Anime is the manga's ad."
I often see this explanation, but is simply wrong.
Anime itself is the main product. That's the standard of the post-'90s anime.
Manga source materials are still necessary cuz it's the easiest way to convince sponsors and greenlight projects.
Also I'm pretty sure a lot of anime is considered but an advertisement for manga in Japan. It's why older anime adaptations had filler eps (Naruto) and stories that diverged completely (OG FMA) because they caught up to the manga.