From a safety perspective, I think not only some legacy carriers, but Ryanair, Easyjet, and Norwegian also do well with "cadets", even those who pay for a type rating, which is borderline pay to fly. Lots of sectors as AQIS Boigu pointed out, and good supervision: A good training captain can teach, with a safety pilot. And unsurprisingly, in many places including China there is a huge demand for experienced Captains, and the salary is ok, even taking various hardships into account.
Now if some operator wants to pair "pay to fly cadets" with "pay to upgrade captains" that does sound like a recipe for trouble. Good luck to Jetstar HK (and Qantas)