My speculations on why he quit

I think multipe reasons combining together made him go "fuck it" so I'm gonna list them from most to least probable.

He was starting to get old and there isn't good money in tech death scene. He needed to start a real career with a real job. He was already practicing mechanical engineering while pursuing Necrophagist, and realized he couldn't keep up both of them at the same time.

We know that his family is conservative from various sources, they probably contributed to this choice as well. You guys probably know the east asian parent memes like "You doctor yet? Talk to me when you're doctor" Turkish parents are also like that. I can confirm that as someone of Turkish descent.

Shitty deals with not only Relapse Records but with the first albums record label as well. These deals probably shrinked his income even more.

I partially blame these money whoring record companies for Necrophagists demise, as Muhammed bad mouths them on several interviews. He was probably pissed off with Relapse Records so much so that he didn't want to release another album with them.

This one is less likely but; I think maybe he wasn't satisfied with the third album when he finished writing. We all know that he is a perfectionist.

I don't know about you guys, but Dawn and Demise is a really mid song in Necrophagist standards in my opinion. Maybe he was dealing with inspiration issues.