I've always though high availability was the more important reason for multiple servers, rather than performance.
Even if you have only two paying customers, they are probably paying for the right to hit your website / service 24/7.
It’s really common to over estimate the cost of being down, while underestimating the costs of resiliency. And in the end if you can’t fully afford the resiliency, you end up with a system that’s more complicated and thus less stable than it could have been had you just accepted a tiny bit of risk.
Productivity of Y Combinator startups goes up. So YC benefits either way.
Edit: fixed url
https://nickcraver.com/blog/2013/11/22/what-it-takes-to-run-...
"The primary reason the utilization is so low is efficient code."
That's why if you get a particularly long thread (1k+ comments), admins will beg people to log out so that the responses can be served from the CDN cache.
Example: American 2016 presidential election https://news.ycombinator.com/item?id=12909752 (1,700 comments)
There are other parts of Y Combinator that still use Cloudflare though.