Long lived unicorns are especially troublesome.
Businesses are like camp fires, camp fires need heat, fuel, and oxygen to burn, if you run out of any one of those three things they go out. By the act of burning, the camp fire can warm the air above it, forcing it to rise which pulls in new oxygen from the surrounding air. They can increase in size which allows them to reach fuel "on their own", and the heat they generate can dry otherwise wet an unsuitable wood and turn it into fuel.
Startups burn cash which as their "fuel", their "heat" is goods and services with which they can change the way a market operates which pulls in more customers giving them more "oxygen" to work with.
As a result, startups and camp fires, share an interesting similarity. The longer you have to work to keep them going the less likely it is that then can survive on their own.
From what is available to the public, it seems that any one of Uber/Airbnb/Dropbox/Pinterest/Palantir would be sold for scrap if they were unable to raise money from private (or public) placement. The key is that they are all losing money so their fires are not self sufficient.
So a really good question is, why not go public?
My experience is that risk is a big factor here. And a number of unicorns have huge risk factors, whether it is bad labor judgments against Uber or sweeping hospitality laws against AirBnB, they have some unresolved 'nut' in their operation which prevents them from showing a risk tolerance that the public would be able to look past. So to evaluate a company that has been around for a while but hasn't shown it can be a profitable business, you have to find the unresolved systemic change that is keeping them unprofitable, and then you have to evaluate whether or not the odds are in favor of that issue being resolved in the company's favor. Which, given their longevity, suggests it is not going so well.