Tornado GR4 is due to slog on until 2025. They'll be needing WSOs (they don't call them Navs any more) for them, and will train a few up until quite close to the end.
And we don't yet know what will replace GR4 - it might still be manned, single-seat or two-seat, unmanned or a mix of all of the above.
And there are other great WSO jobs in the air force - on large platforms, and perhaps on small. Navs can captain Nimrods, and can command Nimrod squadrons, so Navs can have great careers. Support helicopters have been flown single pilot/single crewman, pilot/nav/crewman and two pilot/crewman in the last dozen or so years, for example.