I have commented there, and I will quote my comment here. To clarify, I am not anti-MWI, I am pro- experimental evidence.
It seems to me that you strawman a bit the main objection. Indeed, as you say
MWI certainly does predict the existence of a huge number of unobservable worlds. But it doesn’t postulate them. It derives them, from what it does postulate.
However, this does not answer the objection that
MWI is not a good theory because it’s not testable
if you phrase it the way the MWI opponents usually mean it:
MWI is no more testable than shut-up-and-calculate-only-probabilities-are-real
or than any other interpretation favored by a particular MWI opponent, as long as that interpretation makes exact same predictions as the orthodox QM.
You are certainly right, MWI does not need an extra collapse postulate, it comes out in many possible ways from using the L^2 norm for probability in conjunction with, say, unitarity, or some other equivalent experimentally justified assumption.
Unfortunately your “rightness” is rather hollow, because you still have no definitive experiment that would convince your opponent. And so the argument becomes philosophical rather than physical, as it cannot be resolved by the scientific method.
Needless to say, a QM formulation which would lead to a testable prediction beyond those derivable from the orthodox approach, would be an exceedingly big deal. But one can hope.
MWI is not a good theory because it’s not testable
To which the answer is "its not a theory , its an interpretation. Interpretations arent testable, they're philosophy. Including anti realism".
Sean Carroll, physicist and proponent of Everettian Quantum Mechanics, has just posted a new article going over some of the common objections to EQM and why they are false. Of particular interest to us as rationalists:
Very reminiscent of the quantum physics sequence here! I find that this distinction between number of entities and number of postulates is something that I need to remind people of all the time.
META: This is my first post; if I have done anything wrong, or could have done something better, please tell me!