I think the point of decoupling from the framework is that it's a good proxy for factoring code into a form in which it remains economically feasible to test it with sufficient rigour. It's not because you actually want to switch out the framework at some point to some other named framework. You are likely however to switch out the framework with another version of the same framework, i.e., a Rails major version bump, and we've seen over time that this is a genuine pain point for thousands of companies of all sizes, because typically people couple their code to the framework.

Also, none of this is specific to Rails. I have the same concerns with Yesod, although it's easier to get unstuck in the latter case because of the typechecker.