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Two points:
The MWI/Everett interpetation is the simplest interpretation of quantum mechanics—other interpretations have to add additional assumptions to prevent it from happening.
The most common version of the MWI is actually an interpretation of an interpretation (i.e., Bryce deWitt’s reinterpretation of Hugh Everett's 1957 thesis), but many of those who subscribe to deWitt’s interpretation (including deWitt himself) don’t seem to grasp how it differs from Everett’s. Everett’s thesis makes no explicit reference to multiple worlds—just a single wave function that can be measured in different bases to produce multiple versions of each observer, each of which perceives a different version of the universe. For Everett, the wave function was ontologically prior to the material world, so his universal wave function was a complete explanation as-is. But for deWitt (and for most people), the material world is ontologically prior, while the wave function is just a tool for describing its behavior. So by their reasoning, those multiple perceived worlds must all really exist as parts of the wave function in some sense.
MWI is not simpler than other interpretations. It's more purely mathematical and thus simpler if you ignore experimental physics, yes. But if you consider physics an empirical science, the interpretation has to get pretty complicated to explain why all outcomes of an experiment happen, but only one is ever observed.
It doesn't require fewer assumptions or ad hoc collapse mechanisms, it just moves those to a place where they're harder to see.
But they are all observed, that's the point.
By who? If I measure the spin of an electron in a superposition of up and down, I only ever get one result, up or down.
By the versions of you in each branch.
But which one am I? You postulate that "I" am somehow split into endless copies upon observation, but also "I" am only one of those copies somehow chosen at randomly according to the wave function distribution. So "I" see all outcomes of the experiment but "I" also only see one of them?
This is where it stops being simple to me.
How is the existence of an infinite amount of other worlds a "simple interpretation", that seems like a literal infinite amount of assumptions
Put it this way: is the idea that the stars in the sky are dots on the inside of massive solid sphere more simple then the idea that they're all just other suns very away? The simpleness of a theory isn't determined by how many objects it predicts.
Calling Everett’s interpretation the “many worlds interpretation” is like calling a particle’s wave function the “many particles interpretation”—it’s not wrong, but it makes it sound like you’ve got a multitude of separate things when you’ve really just got one thing of a different kind.
If we sufficiently torture the word "simplest".
Or, you know, use it accurately.