I don't think anyone is arguing that lone founder startups can NEVER succeed; the argument is lone founder startups have a lesser success rate, probably.
And in a game of numbers, it easy to see why YC would want to focus on multi-partner startups over single when there is no shortage of them.
I'm not trying to downplay the importance of co-founders so much as try to show people what they need to be looking for in cofounders as well as what they need themselves. YC has every right to want multiple founders, but for someone who doesn't get in, or doesn't apply, this information could help them know where to focus their time.
And in a game of numbers, it easy to see why YC would want to focus on multi-partner startups over single when there is no shortage of them.
-Zaid