There are security reasons for needing that second pilot if one pilot is there. I can't even go to the bathroom without getting a FA to babysit the other pilot. Also, would need some way to build experience for when someone can be THAT guy in charge of the cockpit, so another guy in the cockpit again. I'm sure I can come up with other reasons too. But single pilot ops is a red herring. So long as you need one, you need two.