But you still have to make a lot of decisions. How do you handle user management ? ACL in a Rails app ? AFAIK Rails doesn't come with a default solution to these problems. What do you think is more important for an application ? User management or turbolinks by default ? The former would make more sense in a full stack framework. In fact both Django and Symfony 2 come with default solutions for this use case.

Finally, the JS ecosystem is moving so fast that decoupling JS from Ruby would make Rails maintenance a lot easier, Rails can't keep up with the front-end ecosystem and its avalanche of "good practice du jour". It's not like RDBMS which are fairly stable and didn't change in 10 years.