Clearly, Sam Altman and Michael Seibel are the most influential and impactful people that drive YC forward. Sam's on the way out, and I wouldn't be surprised if Michael also leaves in 1-2 years. Is YC going to be able to innovate without them?
Furthermore, I would suggest that YC is becoming more risk-averse as it grows (as any organization does). Will that mean worse decision making?
Another point is that the average age of YC partners is growing. Will that hinder them to spot and understand new trends, and pick the right companies?
Now, this is an outside-view analysis. I've spoken to a few YC partners (as part of office hours or at events), but never went through YC myself. I know quite a few people who did, and they seem always most impressed with Sam and Michael.
Edit: Typos