Startup growth is a funnel. Idea stage is at the top, and unicorn is at the bottom. The further down the funnel you invest, the more likely you are to end up with a unicorn.
YC is in the business of investing in startups that become unicorns. YC's brand, and initiatives like Startup School, have gotten them so many more qualified applications. They no longer have to invest in idea stage companies, unless they're insanely promising.
One explanation I have for why it my appear the startups we fund are "far along" at demo day is that the startups moved so quickly during the batch. For example, the startup Inokyo in the current batch recently put out a video of the autonomous store they built in Mountain View: https://techcrunch.com/2018/08/16/inokyo/ When we funded this startup 3 months ago it was for a different idea and so they quite literally started from scratch.
Startups moving fast should hopefully seem far along by demo day, irrespective of their starting point :)
A hard number would say more than that phrase :)
Can you give specific examples of startups they have funded that became unicorns? The only two that come to mind are reddit and airbnb, and both are fraught with criticisms and legal issues over their business practices. Maybe there are more, but that's still an insanely low percentage of the total startups they have funded...
sadly it has become a business. I guess we need a new old YC.
It is, literally, a multi-billion dollar private equity firm.