Fwiw I remember flying a pretty empty AirBerlin 321 with passengers only at the extreme ends of the cabin. The emergency rows all remained empty.
There should be some commonsense applied. If there's only twenty pax on board with at least 3 FAs, why on earth do all the overwing exit seats have to be full of ABPs?
They could all get out of one of the main doors in a few seconds, without needing to use the OWs.