All good points, but on this one I am going to call you. Although from a pure population viewpoint you are correct, it would be a statistical lie to assume that the correct number to use would be total population divided by number of records sold.
First of all, you need to account for the fact that certain age groups buy music and certain ones do not. There are a lot more 60-80 year-olds now than their were during Elvis/Beatles era as a percentage of the population and they tend to buy less music than 18 - 35 year-olds who were a larger portion of the population (baby boomers) when the other artists were recording. In addition you need to make an allowance for timeframe. My guess is running the numbers would have Jackson closer than you claim.
I don't dispute your overall point, but as a conservative, I think it is important to be honest with numbers and how they are logically derived.
But, you left out the fact that Elvis was forbidden by many parents. And, that kids did not have buying power in the 50’s.
Slick Willardbots don’t have much credibility to argue about honesty and logic. ;-)