I'm curious: how did you extrapolate from Wil's points on how to be a successful YC applicant that he also thought all YC companies are going to be successful?

I'm curious: how did you extrapolate from Wil's points on how to be a successful YC applicant that he also thought all YC companies are going to be successful? These are qualities for successful founders, not successful companies. There are a myriad of reasons why a startup might fail. Having founders with the traits outlined in this essay helps prevent the founders from being one of those reasons. That's why YC looks for these traits. Since YC screens applicants, they can select the ones they feel are best suited for success. Based on past experience, those with the traits in this essay are the most successful. Minimizing the founders as the source of failure means YC might have more successes, on average. As pg said, even YC would be surprised if their "success" rate is more than 50%. Being accepted into YC doesn't guarantee success... Creating a successful company is hard!