If TC is 2k, NQ pay easily doubles or triples that once qualified.
This incentivises a SG firm to let go NQs and 1-2PQEs. Especially firms with ready supply of trainees each year.
Bosses don't have to outrightly fire. Just make things hard for them so they cannot tahan and jump firms early. Or switch from practice to be civil servants, minlaw execs. I'm sure you know peers who only lasted about a year post NQ and the firms they're from are considered targets, even B4.