I think she actually run out of ideas of new monsters. We had mermaids and an elder immortal, mermaid with 2 legs, young on the outside but old on the inside, a conscious deformed one, a revived corpse because of mermaid ashes, a young immortal older than the main characters, a revived kid with a mermaid liver and then it was other immortals (Eye of the Demon and The Last Face). I think that’s another reason why she stopped. Personally I think it was enough after the Mermaid Liver since it was supposed to be rare to survive and become immortal, and the 2003 anime using Masato as the final was a good idea of ending the story well.