But that's due to functional limitations, is it not? As you say, "at the moment."
If not being able to step outside of the universe and look around is a functional limitaion, then sure. There is no way to know 'at the moment' if the theory will ever provide a testable hypothesis. Even if you find another universe, you still have the problem of it's origin.
It seems to me there is a big difference between presenting a scientific theory that cannot at the moment be tested but theoretically could later on, as opposed to presenting a "theory" that can never be tested.
I would venture the opinion that it is unwise to declare that a theory 'can never be tested'. I prefer the term 'highly unlikely'....just in case there are other surprises out there, like, say, mirror universes with different laws of physics..............
But I do take your point.
Even if a theory cannot be tested, if it is suggested by and compatible with the evidence, even conceptual evidence such as mathematical models, there is still more basis for that theory than one that did not arise from the evidence.
That's what the ID crowd say. Btw, if you have evidence for multiverse theory please let me know. Jumping back to ID for a minute, there are some good mathematical models that show how an irreducibly complex system cannot evolve for example, with complex details that perfectly align to the nth degree, but guess what, that doesn't mean there are such systems in nature.
Mutliverse theory is pure speculation, of the most sophisticated kind mind you, but speculation nevertheless. It's a pity we don't have a way to test it. Maybe we never will. For now it is a creation myth, as Robert Stewart pointed out. So, w
hat is the difference between belief in a multiverse and belief in God? Not much that I can see in this context.