At the selection phase there's very little to choose between those who go Pilot and those who go WSO. Some who are selected WSO join having applied Pilot and retain a deep seated desire to achieve that goal, others accept that the role they have been selected for is extremely challenging and get on with life. Speaking as a retired Nav, I rarely took the opportunity to pole the jet, it wasn't what I was being paid to be good at so it bored me (although I did once land an f3 from the back seat
). As has been said there are ocassionally chances to cross over and many of those have done extremely well (the Typhoon force seems to have loads of ex Navs in exec slots). however, it would be a foolish person that accepted a commission as a WSO expecting to get a crossover. Service needs rule, not personal goals.