I sympathize with Nomura. It's hard creating compelling villains and antagonists in a game. Especially one where people need or want more to the story like with Kingdom Hearts. So it's easier to reuse Xehanort, who's been a staple in the series since the beginning, rather than introducing someone else down the line as the true puppetmaster, especially without clear and obvious hints before. People would've hated that even more. To this day people still complain about how Kaguya and Zetsu usurped Madara as the villain at the end of Naruto.