That's not sufficient to serve a Rails site though.
Rails is a great platform for building a custom web app, but it's a terrible platform for building an open-source blogging platform. It simply moves too quickly and breaks things too often to be useful for the demographic that uses WordPress.
When you're talking about web code with such an astronomical deployment to developer ratio, the advantages of PHP are magnified by several orders of magnitude. Even the perfect WordPress replacement written in Rails with elegant, featureful and performant Ruby code digitized by God's own hand is by definition a niche product that can never hope to compete meaningfully with WordPress.