If they aren’t working for the firm by the time they leave university then I don’t think they ever will. Eugenie, Bea, Zara and Peter are all adults who attend the occasional family event but they have lives and priorities of their own and haven’t been trained or prepared to be full time royals. People wouldn’t accept more royals being added to the payroll without someone getting cut anyway.
The way I see it happening is that people will continue to work but when they pass away their children won’t take up the mantle. If you think about it, Anne is unlikely to stop at all so I can see her going for a good twenty years yet and it’ll be much the same for the others. By that point George, Charlotte and Baby Cambridge will be able to work for the family anyway and George and Charlotte may well be married so there will be people to pick up the work.