If you've never seen any evidence of that, you've not looked very hard: This is what every supermarket does for their filler jobs, and to a lesser extent for their till and other store facing jobs as well. If you're over 18, you have to be very lucky to keep your job (or more realistically, to keep being given work) in such a situation.

Outside of supermarkets this sort of practice is also pretty entrenched in a lot of small-medium IT businesses: They have a sizeable portion of their workforce as trainees or whatever (they may be underpaid legally, so they almost always are) and don't hire them after their time is up (ever, as a rule). New trainees are then cycled into the slots that are now vacant.