You don't need partners great at scaling profitable companies from 100 to 500 employees.

You need partners who know how to help a company go from two to ten employees and raise a first round of funding.

I'm a part-time partner, and I run a company with >500 employees, but I basically draw entirely on my first 2-3 years of experience when giving advice.

Don't you think such people would be interested in investing on their own rather than part of YC as their partner? I don't know the revenue sharing approach that YC uses with their partners but I don't believe that it is good enough to make successful entrepreneurs who built a company with >500 employees to join them. Someone with more insider knowledge can comment.

Why would someone that accomplished join this group? Peter Thiel started the Thiel Fellowship to do his own program.

Why does it have to be someone who both created a business and scaled it to > 500? The skill-sets on founding vs. scaling are different and very few leaders possess both and had the drive to scale the company they founded to be that big.