It means that Sega have to pay more and more royalties to others, making the series unviable to produce now.
He does the characters creation, design the package art and a bit more, but the designs inside the game are drawn by another artist. The digital colors ones are drawn by a different artist.
Sega have to pay Studio I.G. for the CGs and animations, Tanaka Kouhei for the sounds, Red for the production and the character designer for the characters... that's why Naka Yuji was not happy with the series.