Biggus,
Good points. Here's my 2 cents worth (about 1p given today's exchange).
There is a problem with junior ranks retention. Hence the FRI for Regiment and Firefighters. But S/L Aircrew CS types, by that stage, have had a huge investment into them, and to get someone else up to that level of qualification/experience would take the same amount of time/money, both of which the RAF don't have.
I don't mean to be blaise about this, but the workforce pool outside the RAF offers more opportunities to recruit someone into the junior ranks and get them to the point we need them to fill gaps. Not so easy to pick people off the streets and have them at Sqn Ldr level within a few years.
While I'm sure we all like to volcalise the value each one of us is to the service and the work we do, there is a pecking order and pyramid of abilities, to which you presume the higher up that pyramid you are, the more difficult it is the replace you with someone whose qualified to do so. As a Flt Lt pilot, I'm well aware of my place, and if retention is an issue for a branch / speciality I'm unable to fill myself, I recognise that fact and don't feel the need to turn the issue around to "What about me then ?"