The idea of a commons as a shared resource that must be maintained through collective action is still a useful metaphor for our global environment. Just because Hardin's scholarship about medieval commons was bad doesn't mean our global world is not in itself a commons as the metaphor described it, and a useful thought experiment based on all the scholarship and debate that followed. And everyday people in a comment section who aren't environmental professionals aren't going to know the word "externality". So not that useful outside of a limited audience