Put yourself in the shoes of an 18 yo kid straight out of high school. An opportunity to do an 18 month course to then be flying all over Asia as a jet SO by the time they’re 20. And if they get sick of that, they have first dibs at positions within QF.
From what I have observed there are not that many 18 yo straight out of school cadets and the ones that are, are immature in the way they conduct themselves. The best cadets I have flown with are those with some previous work experience. I do agree with you that they get sick of being a Jetstar pilot because they start to realise that airline flying is not all glamour and cool sunglasses. I'm not sure though why you think they won't eventually feel the same way after the excitement of being a QF SO wears off. I'm also not sure why you think that they get "first dibs" within QF. They get the same opportunity as any other QF Group pilot. In fact they get less opportunity until they achieve the minimum PIC requirements.