That's kind of the problem. We don't view companies that go 12 months without revenue as high risk and/or failed investments anymore. How long can companies like Uber continue to raise capital to operate when they're only generating a fraction of the revenue now and investors won't accept the long term risk?
As long as money is cheap, the rich (VCs, Angels) will throw cash at you to satisfy their tax liability. A unicorn going public is just a side benefit.