Yes, you could derive the horizon stuff from special relativity, but to construct an asymptotically de Sitter spacetime you need general relativity. Anyway, that wasn’t the original issue. “no collapse at intermediate scales is a good hypothesis and maybe wrong for this specific reason” is one possibility, the likelihood of which is currently hard to evaluate, as it extrapolates quantum mechanics far beyond the domain where it had been tested (Zeilinger’s bucky ball double slit experiments). The nature of the apparent collapse is a huge open problem, with decoherence and Zurek’s quantum Darwinism giving some hints at why certain states survive and others don’t, and pretending that MWI somehow dissolves the issue, the way Eliezer tells the tale, is a bit of a delusion. Anyway, MWI does not make any predictions, since it simply tells you that the feeling of being in a single world is an illusion, without going into the details of how to resolve the Wigner’s friend and similar paradoxes. See Scott Aaronson’s lecture 12 on the topic for more discussion.
Yes, you could derive the horizon stuff from special relativity, but to construct an asymptotically de Sitter spacetime you need general relativity. Anyway, that wasn’t the original issue. “no collapse at intermediate scales is a good hypothesis and maybe wrong for this specific reason” is one possibility, the likelihood of which is currently hard to evaluate, as it extrapolates quantum mechanics far beyond the domain where it had been tested (Zeilinger’s bucky ball double slit experiments). The nature of the apparent collapse is a huge open problem, with decoherence and Zurek’s quantum Darwinism giving some hints at why certain states survive and others don’t, and pretending that MWI somehow dissolves the issue, the way Eliezer tells the tale, is a bit of a delusion. Anyway, MWI does not make any predictions, since it simply tells you that the feeling of being in a single world is an illusion, without going into the details of how to resolve the Wigner’s friend and similar paradoxes. See Scott Aaronson’s lecture 12 on the topic for more discussion.