Thanks for the cordial discussion, but I think the physicist you mention has it right. "Prediction" of phenomena that can never be observed seems like an oxymoron.
Yeah, but there is one practical application, if we ever conquer the stars: if we send colonists so far away that they eventually cross the boundary of our observable universe, we care a great deal about their continued survival, even though we will never observe it directly.
Other unobservable stuff may or may not be incredibly important. (Though at a first glance, QM interpretation isn't.)
