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The assumption of “the existence of fundamental ‘observer’ systems” takes us out of the agreed purely technical discussion. Such systems are not implied by the universally valid and complete quantum theory.
Nevertheless, even if it were applied, my argument does not change: Bearing in mind that quantum theory equally applies to every possible decomposition of the Universe into subsystems, the ‘fundamental observer systems’ (FOSs) must exist for virtually all Universe decompositions.
Please notice that the decompositions I have in mind are such that no subsystem (and therefore the FOSs) appear in the alternative decomposition–otherwise the variation of decompositions lead merely to ’emergent structures’. In the hydrogen atom mentioned in my previous post: if there is an FOS in electron+proton (e+p), then there should be some another for the center-of-mass+internal-degrees-of-freedom (CM+R); of course, there is no ‘electron’ in the CM+R atomic structure and hence an FOS in the e+p structure is not reducible to a FOS in the CM+R structure.