What does one do if one simply cannot believe in the supernatural because of the lack of evidence?
I think you’re being a bit uncharitable with the consequences of nihilism — or to be more precise, metaphysical naturalism. I haven’t met very many people, even non-religious people, who truly believe that they can just do whatever the hell they want because nothing matters. If I met people like that, I’d cut them out of my life. Even if you don’t have an intrinsic motivation to do good, you just can’t escape the fact that if you’re an asshole and not a sociopath, your life will probably suck.
The truth is that it’s possible to be a well-adjusted, loving human being while also believing that life is objectively meaningless. (Though ironically, in my anecdotal experience finding happiness as a metaphysical naturalist is harder if you grew up with religion and then left it, like myself). Today we can draw from stoicism, optimistic nihilism, secular humanism and others to help us along the way.
All this said, I do not know whether a worldview divorced from objective meaning is better for building a healthy society. I suspect it’s more difficult without religion, which is one of the reasons religion is so widespread.