I like your thoughts of an full stack designer but I don’t think the designer will have the time or effort to master the Frontend as well in a project. The frontend is moving too fast for a designer to keep up with best practices and new ways of writing code.
And another perspective is that what will happen when a Senior Frontend developer takes over the code and need to refactor because it is not written exactly as it should be in his/her mind? I think there would a clash there which will be time consuming.
Great article by the way.