The implicit assumption is that MWI can do "at least as well as Copenhagen, everyone can". Except that this statement is completely and totally wrong. ...I used to think that MWI was an interpretation, reproducing Copenhagen predictions. Then one's belief in it is a matter of metaphysical preference.
The MWI fairy-tales are among the top reasons why I lost my respect for many physicists who have done some nontrivial technical things. But they're just lousy thinkers if they can't figure out the lethal problems with the MWI above – in fact, they seem unable to figure out even 5% of those things. How much smarter the founding fathers of quantum mechanics were. They were able not only to understand them but to discover them in the first place – which is an achievement greater than a mere understanding, by many orders of magnitude.
But it is not. MWI predicts nothing, and has no merits at all.
I agree with Lumo here. There are seemingly-competent physicists who endorse MWI, and I have lost all respect for them. MWI is such complete foolishness that anyone who endorses it should not be taken seriously on any scientific matter.
His arguments are somewhat different from the ones I have given here, but the end result is the same. There is no way to turn MWI into a useful theory. It is just a weirdo unscientific fantasy.