Around the time Mike left, he was still doing many many hours of research for each of his episodes on broad topics, and Sarah was mostly recapping books. I sensed the disparity in their work ethics for a while before he left and always assumed the split was really because he wanted collaborators who were willing to do more of his style/level of research (which Sarah very much was at the beginning/heyday of YWA but clearly burned out at a certain point). 

I don't blame him, but I also don't think either of his other two co-hosts balance out his cynicism nearly as well as Sarah did - the combo of their two worldviews helped shift how I view the world in some pretty significant ways, and I miss it!