Cam Rayner is simply not "a jet in all levels". In fact, up until the U18 Champs he had underperformed. His school footy form has been underwhelming, and his TAC Cup form early did not show much midfield capacity. Even still, he doesn't have the aerobic capacity to play majority gametime in the midfield, which follows his poor beep test. I, personally, don't see him as a midfielder at all, not until he's spent at least 3-4 years increasing his endurance a lot, and consider him to be an excellent medium forward, but his questionable endurance is still concerning.
The beep test is still very reliable for determining players who have excellent fitness and who has poor endurance. For those in the middling, 13.1-14.5 levels it doesn't show much to separate them, but I do not agree that some people 'just can't do it'. That's a fallacy. Especially for AFL players, though some may not take to the harsh turns and gradually increasing tempo, there is no excuse for a player aspiring to play at the highest level to not at least record a 12.5.