Because there's already social safety nets those people can use. And yes, absolutely, there are probably going to be groups of people who are barely working now that will stop, but given there's already means to not work, I can't see this being a huge group, and really, this is an optimist / pessimist face-off, which is sad, because that's how we probably see the possible outcomes too. The only way we can know if it'll work or not is for someone to try it - which, thankfully, YC is.