MWI and Copenhagen do not make the same predictions in all cases, just in testable ones. There is a simple program that makes the same predictions as MWI in all cases. There appears to be no comparably simple program that makes the same predictions as Copenhagen in all cases. So, if you gave me some complicated test which could not be carried out today, but on which the predictions of MWI and Copenhagen differed, and asked me to make a prediction about what would happen if the experiment was somehow run (it seems likely that such experiments will be possible at some point in the extremely distant future) I would predict that MWI will be correct with overwhelming probability. I agree that if some other "more complicated" theory made the same predictions as MWI in every case, then K-complexity would not give good grounds to decide between them.

I guess the fundamental disagreement is that you think MWI and Copenhagen are the same theory because discriminating between them is right now far out of reach. But I think the existence of any situation where they make different hypotheses is precisely sufficient to consider them different theories. I don't know why "testable" (meaning testable in practice, not in theory) was thrown in at the last minute, because it does not seem to appear anywhere in the rest of the post.

If instead you are asserting that MWI and Copenhagen make the same theoretically testable predictions, then I disagree as a matter of fact. MWI asserts that interference should be able to occur on arbitrary scales, in particular on the scale of an entire planet or galaxy (even though such interference is spectacularly difficult to engineer and/or will have a very small effect on probability amplitudes), while Copenhagen seems to imply that it cannot occur on any scale larger than a human observer.

I wouldn't be surprised if I am wrong on that question of fact, and it would certainly be good for me to fix my error now if I am.