There's some truth in that, but there is more to a manga than just marketing, and some mangaka do retain creative rights. Masami Tsuda, the author of KK&J managed to shut friggin Ano Hideaki down; there was no 2nd season of the series because she disagreed with the direction he was taking it. The only person in anime whose name carries more weight than Hideaki is probably Miyazaki himself. And she just told him, "No, I don't want to work with you any more."
Masami Tsuda has more balls than Kishimoto.
Her career didn't end after that, either. She's still publishing manga.