They would get a different Oyster Card, registered to them, which gave them cheaper fares. The idea of a Key Workers Oyster Card, valid only with a suitable staff pass, and higher fares for everyone else, isn't fantasy.And I don’t believe the argument that key workers and such could receive lower fares - how would this work in practise? TfL can even register railcards to your contactless card
(I appreciate that there is an issue with passengers who prefer Contactless to Oyster but the same applies for railcard discounts, children and similar.)