Well he did walk out on FNIA Studios as well way before now and they didn't get the chance to release the 4.0 update and had to be delayed. I actually got interested in both FNIA and FNAF after I started playing FNIA studio's visual novel. So, it's not just Yuuto he walked out that close to releasing the update. Which begs the question to be honest, who else as both times were close to completion? To be honest the only thing I question is the integrity.
If he don't want to use AI that be fine, but. If Yuuto still feels this way, then the absolute last thing he could do if he really don't want to cancel it is just pass the torch as Yuuto has good ties with FNIA Studios. As this the final idea after the AI is pass the project to FNIA Studios if Yuuto needs to take a break. I lost respect for Rex after he left them both in a cheap ass manner just because he wanted to participate with FNIA RX and FNIA Reborn.