I think "private monopolies are worse than government ones" is probably true in my experience as well. Although some of this is the subjective experience of having to pay money to be treated badly.
I think this makes me believe more strongly in competition as the main reason why the USPS is comparatively well-run.
I would still expect private monopolies to be run more cost-efficiently than government ones. Although I'm not sure about cases like utilities where their profits are directly tied to their costs by government regulations.
For it to be a formal claim would require us knowing more physics than we do such that we would know the true metaphysics of the universe.
You are correct that I used Church-Turing as a shortcut to demonstrate my claim that MWH is computable. However, I am not aware of anyone seriously claiming quantum physics is non computable. People simulate quantum physics on computers all the time, although it is slow.
I'm inclined to view your description as a strawman of MWI
I don't think it's quite a strawman, since the point is that MTM is literally equivalent to MWH. In math saying "A is isomorphic to B, but B is easier to reason about" is something that is done all the time.
but it's also not an argument against MWI, only against MWI mattering to your purposes.
I like the Mathematical Universe Hypothesis for simplicity and internal consistency, but it seems like we're assuming a lot. And it's not as simple as all that, either. Where do we draw the line? Only computable functions? The whole Turing hierarchy? Non-standard Turing Machines? If we draw the line at "anything logically conceivable", I would worry that things like "a demon that can jump between different branches of the multiverse" ought to be popping into our reality all the time.
If we want our theory to be predictive, we should probably cut it off at "anything computable exists", but if predictability was our goal, why not go all the way back to "anything observable exists"?
The MTM model is completely non interacting
The MTM model is literally computing the same thing as the MWH. Specifically, suppose for a human brain I compute the events observed by the same human brain. Granted, this requires solving both the easy problem of consciousness and the grand unified theory . But I don't think anyone here is seriously suggesting those are inherently non-computable functions.
I suppose a reasonable objection is that the shortest program is MWH, since I don't have to determine when an observation happens. But if I ask for the fastest program in terms of time and memory efficiency instead, MWH is a clear loser.