I have this question about MWI I always wanted to ask but never dared to! It could be that I just don't know enough physics to understand the answer, or the question!
Anyway, here goes: What is it that differentiates MWI from a physical theory (let's call it 'pure random') which is equivalent but where, in every instance where MWI asserts that the universe spawns/splits up (Schrödinger's cat-esque situations), this theory simply asserts that the universe flips a coin and randomly1 decides which of the possible universes should be realized. In this theory, the other forks are never realized, so there's always just one universe in one state making a bunch of random choices how to proceed.
To me this theory seems less objectionable than a theory postulating an enormous amount of parallel universes that can't see each other anyway. So my question is, is there any physical reason to prefer MWI over the 'pure random' theory (and what is this 'pure random' theory called anyway)? After all, the parallel universes in MWI are inaccessible anyway, so it doesn't seem to have any consequence they exist. It's just another way of expressing randomness... and it doesn't even seem MWI can fully do away with randomness anyway. (After all, why am I experiencing this particular universe, and not one the many other ones MWI asserts exists? To me it seems I'm experiencing a random one among the possible ones.)
I'm wondering if my 'pure random' theory is really the same thing as what MWI-believers talk about, albeit they use a more colorful language and that's why it gets presented this way in the popular press. But on the other hand many presentations of MWI (including Wikipedia) seem to imply these multiple universes really do exist, and that Everett was of this belief.
Apart from the obvious consequences to things like quantum immortality (which Everett also believed in, further underlying he believed in the existence of these parallel universes), are there other observable/measurable differences between MWI and pure random or other reasons to prefer this?
1 The probability distribution would be to reflect the event in question, just as I imagine that MWI would in some situations split up several universes with various properties, and the 'concentration' of a given property reflects the probability of measuring that property... I hope it makes sense!