Most of the time one competent 10x engineer could indeed build out all the special features that a company needs. The problem is that one person generally doesn't know all the best practices required of every problem domain, so they may implement things poorly. Furthermore, the one dev thing really doesn't scale up. As soon as your product gains any critical mass and you have an outage then the platform is hosed because no one knows how to fix anything except MAYBE that one engineer who implemented all the special features.