Without any data on usually vs. unusually a debate is lost anyway! FWIS there are enough stories of successes from solo-founders to contradict your theory - flat on its face.

> can't even convince one more person to join you

... is not the same thing as don't want another person in the founding team. I still do not see how that 10 year old logic of a co-founder must-have is going to hold good in the coming next 10 years. I may individually fail but times for startups have changed, haven't they?

Also it is still a bad deal for most entrepreneurs (especially those who are outside) to let YC become the fulcrum of startup destiny - for it has almost become a monopoly and a monopsony as I'd mentioned earlier. It doesn't matter whether YC likes it or not.