I bet it was less about fanaticism and more about time. Previously, he had worked full time and made money. He wasn't able to work after the illness (I think I read somewhere that he was rehired by his old company, but only lasted a few weeks).

TempleOS wasn't the first thing he worked on after becoming ill, he'd previously been working on a three-axis milling machine. https://en.wikipedia.org/wiki/Terry_A._Davis#Onset_of_illnes...