You'd have to know their grand vision to decide whether they can become real businesses. And even then, you'd have to buy into it.
I'm confident YC is not investing in any company without a shot at becoming a multi-billion dollar business.
There are a fair number of zombie startups in Silicon Valley that haven't yet exhausted their funding. Some even have enough revenue to keep the lights on. But it doesn't mean they're anywhere close to having a thriving business that will be able to sustain itself and grow meaningfully over time.
When evaluating startups, alive/dead is a lazy means of analysis. This is particularly true in today's funding environment, where companies can raise seven figure seed rounds and establish a fairly long runway very early on. The real question is how many of the startups funded today will ever realistically be in a financial position to return capital, and how much, to their investors.
Edit: Not trying to discredit what these start ups have done so far, it's very hard getting to that point alone.