I believe median time to unicorn status is now about 7 years, so this is not as uncertain as you think. In other words, 50% of YC's 7-year old unicorns have already showed up. The number is higher for older companies.
Dropbox and Airbnb were superhot companies within 2 or 3 years after YC. My first Airbnb booking was in 2013, and that
I felt like a late adopter.
Yes, but average time to unicorn status has been increasing since the time of Dropbox and Airbnb. I assume you can draw more inferences from companies that are earlier-stage, too.
If you read their tweets, YC's message is that their companies are growing faster than ever