Thing is, though: If Everett-like theories “require unobservable other branches of the UWF” then they introduce a metaphysical entity for no good physical reason at all, solely in order to salvage a causal interpretation which is likely not salvageable in this domain anyway.
At least that’s what Bohr thought when he diagnosed the situation as such: “Indeed the finite interaction between object and measuring agencies conditioned by the very existence of the quantum of action entails — because of the impossibility of controlling the reaction of the object on the measuring instru- ments if these are to serve their purpose — the necessity of a final renunciation of the classical ideal of causality and a radical revision of our attitude towards the problem of physical reality.”
The thing is this: Causality is, as already pointed out by David Hume, a concept invented and used by the human mind in order to make predictions, not a universal property of an independent reality. As such, there will be limits to its applicability, and the standard interpretation is that quantum physics has reached that limit.
As long as it introduces no new predictions that differ from standard quantum mechanics, Everett- and Bohm-like theories seem more like an ideological metaphysical superstructure intended to salvage a philosophical outlook which seems to be untenable anyway.
Of course, the moment they do come up with different predictions is the moment they start becoming interesting as physical theories. The Bohm pilot wave theory did come up with some different predictions (and are, I believe, disproven). The decoherence/Everett theories not yet. If they don’t result in different predictions, standard QM will be preferred by most physicists because it is simpler and more elegant.
