[pontus]

You're right that you can't just count the number of postulates naively because there is generally not a well defined way to do so. A great example is the one you give: three laws of motion vs one law of least action. However, if I told you that it was possible to reproduce all of mechanics with only the first two of Newton's laws, then surely you'd agree that there wouldn't be a need for the third law and in that sense the new system of postulates would be simpler.

In other words, because MWI is obtained by removing a postulate from the usual formulation of QM, I think it's fair to say it's simpler. If, instead, MWI had been obtained by formulating all of QM in some other distinct framework where there was no mention of wave functions, measurements, or the Schrodinger equation etc, and it had one fewer postulate, then yes I would agree that you can't arbitrarily say that it's simpler.