Rotate.
PAS, and this year PAS plus FRI is purely about retaining specific individuals for (a minimum) of 5 years - nothing more. Why would those who have already decided to accept PAS be offered anything? They have already been retained; there is no need to spend the money. Those who decided to go for a career did just that; decided to do it. I suspect that PAS offers have been targeted at those with (unsurprisingly) pertinent flying quals where there could be a shortage, A2, QWI for instance. It's about retaining specific individuals with required quals - no need to retain someone who sits behind a desk with no specific skills in their back pocket, the reality is that they can be replaced by someone else who will do the same job to an adequate standard very quickly.
Agree with you about the devisiveness of FRIs, but that has always been the case.