The original idea is probably the least important thing about most startups. There are cases where the founders have been working in a domain for many years and

The original idea is probably the least important thing about most startups. There are cases where the founders have been working in a domain for many years and start a company with a very specific pain point in mind. Those are the exceptions. Most successful founders have an initial idea, talk to potential customers, and find a related and sometimes completely unrelated thing that people want more and/or that excites them more. It may yet turn out that having an idea is an indicator of something that is currently unknown but important, and that cannot be filtered for otherwise. If this happens, it will be apparent to YC soon. Meanwhile they are innovating, taking risks, and trying new things - which is exactly what I would expect from a group of people teaching others how to build better startups.