There's two factors at play here.
- Management are unlikely to ever let Nomura helm another mainline FF because he took too long with FFXV and they now want him to focus on developing games in the mainline Kingdom Hearts series.
- Nomura may not even want to direct another mainline FF because of his troubling experience with FFXV.
1. We don't know all the behind-the-scenes story of FFXV's development, and how much was actually Nomura's fault. But we do know that he created a successful franchise and directed some very well received games. The management also know this.
2. It's not uncommon that people change their minds on such matters. Kojima has said four or five times that he was making his last MGS.
So I wouldn't rule him out completely just yet.