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.