Pretty obvious, I'd have thought.
Hiring cadets does nothing to move the market as it doesn't affect anyone in the market. Neither does internal promotion. No FOs moving from turboprops to jets so no piston drivers moving onto the TPs, and no instructors moving into piston twins. No ex-military getting employed in civvy street either.
No movement anywhere. No cross-fertilization at all. And we know what effect that has on a closed-cycle gene pool, don't we?
Cadets only = market remains bogged down & static.
Ditto internal promotions only and no DECs.
Closed gene pool = ever increasing proportion of mongs.
Hmm!