The reason why Rails is going to continue being awesome even in the API-dominated web apps of tomorrow, is the asset pipeline. There's just nothing else out there that manages your assets quite the same way. In fact, the asset pipeline makes it dead-simple and even efficient to program big, high-performance apps in CoffeeScript. Sure, it doesn't force asynchronous behavior, but I feel that's more of a feature than a flaw. Sometimes, asynchronous calls are just not possible (like in the case of Authorize.Net's API), and you need to build something a little more robust, and easy to test when things go wrong. Therefore, I feel Rails is actually the best choice for an API backend, because it has so many drop-in monitoring tools available as well as ways to enhance performance by dropping frameworks you don't need, plus you can code asynchronously with it but you don't have to.