I agree with you that the FAA has done a poor job of explaining what electrical interference there is, and what it's effects are. I also agree that avionics gear is not so sensitive that an iPhone's connection should harm it. But your argument that the pilot should deal with problems like this in-flight is just a tad bit on the silly side IMO.
The typical pilot's mindset is, "Fly the plane but, when a problem occurs, stop focusing on flying the plane and solve/identify the problem. Once the problem is solved/identified, get back to flying the plane." You're effectively wanting to change this to something more like, "Fly the plane but, when a problem occurs, stop focusing on flying the plane and solve/identify the problem except if the problem is 'electrical interference'. If that's the case, ask yet again that people turn their phones off. Hopefully they'll listen this time. Wait for all of the people to comply and, once the problem is solved, get back to flying the plane."
It just sounds silly, doesn't it? I think we'd all agree that, if there's a problem with electrical interference during flight, we want the pilot to be flying the plane rather than having him/her wait for the passengers to do anything. Again - I'm not arguing whether this is right/wrong; I'm simply pointing out that the logic of having a pilot wait on passenger behaviors before being able to continue doing his/her job is a bit silly.