And why are you so insistent that it would forever be out of the civvies' learning ability? Were you born with some special insight into AAR, or was it taught to you? Why would it be so impossible for civvy pilots to do the AAR course, hypothetically? Some might not be up to it, granted, but to make the statement that none of them would be capable is ridiculous.
Think of it this way. An AT/AAR pilot needs to be competent in aircraft operation, AT role operation and AAR role operation.
There's no doubt that any airline pilot could meet the first 2, but the 3rd is an 'unknown'. The course needed would perhaps be longer than converting an existing AAR pilot to a new AAR aeroplane and there would be
some risk that the airline pilot might be unsuccessful as you say. But at the end of the day it boils down to one thing alone - cost. If it costs more to train an airline pilot to become a competent AAR pilot than it does to convert an RAF AAR pilot to the new aeroplane, the military being as it is a 'price-sensitive' customer is going to take the latter option.
The 'fATPL course' includes rather more than the RAF ME course does. But you have to bear in mind that RAF candidates are given pre-entry screening, whereas in theory anyone with enough money can buy their own 'fATPL' and 737/320 TR. However, the new MPL course does require end-user airline involvement - so it's in the financial interest of the airline to pre-select its 'MPL' candidates.