Egdg,
Retaining some experienced navs on PAS seems very sensible to me. There's still a requirement for them until the GR4 OSD, and beyond that their specific skill set doesn't just disappear. Look at the many ex-Tornado navs performing very well on Reaper.
Plus, some of the dreaded desk jobs will require any aircrew, not specifically pilots, so there are plenty of routes to employ them and use their experience for many years.