There are several big issues within the OP's question.
'Is the selection process valid?' assumes we have a validity test. We don't. No one knows just how to spot FJ aircrew from a selection of teenagers, mainly because it's not like anything they've already done. It would be a lot easier if we did know.
Next is the assumption that what is required is constant but it most likely isn't. The combination of required motor skills, awareness, technical knowledge etc has varied over time and between aircraft types and roles. In simple terms, FJ aircrew personal requirements are a moving target and this means previous experience of selection and training may not be usable.
Then there's the financial aspect. It may seem like failing a large proportion is indicative of a faulty system, but that may not be the case economically. How much flex do you give students, and when?
Lastly, there's standards. As many have commented, a pass today may be a fail a decade hence, and v.v., and from the point of view of national defence, they may both be right.