Not completely true, Amano did the initial character designs for FFIX and he also helped with some of VII's designs as well. Sephiroth is collaboration between Amano and Nomura. Noumra style was just easier for 3D cause it was simpler in overall design. It didn't help that his non-FF work brought him more recognition out of Japan in the mid-90s and he did several world tours with his art galleries.

Though I agree that Amano did a lot for the series and that a part of the appeal of the early games is trying to capture the allure of his artwork that inspired the games to begin with.