Success is rarely so rapid that you can't throw money at a problem temporarily and build something more robust. No one is advocating for a single server runnin

Success is rarely so rapid that you can't throw money at a problem temporarily and build something more robust. No one is advocating for a single server running in your closet, but a large and well funded PaaS can handle any realistic amount of growth at least temporarily, and something like Heroku is big enough (and more importantly, owned by a company big enough) that shutting down without notice is not a possibility worth considering.