I think that's kind of the point though - in a free market with sufficient competition, distribution channels would be forced to compete on the quality of their review processes and users would gravitate to the marketplaces that have the best processes and hence the best reputation. So it is not clear that this is true mathematically: you just need one alternate channel with better review processes and for enough users to switch to it for there to be less malware distributed overall.

As it is, there is only one channel and no competition, so we don't know what the malware situation would be like in the absence of Apple's monopoly.