I think it is more a matter of perspective than not having enough new unicorns. Every "Unicorn" looks like every other startup until it doesn't, that being said there are a few things all successful startups have in common and a plurality of things holding startups back from succeeding.

Rather than make a list of things that I am no means an expert in, I will just say that YC seems to find a lot of startups with potential and if your a founder looking for motivation maybe this book might help?

https://www.dummies.com/business/fundraising/venture-capital...