Whats going to stop all us that joined more than 4 years ago from leaving though?
Nothing - arguably the pension. It's been mentioned a few times on this forum that it's likely that since the dawn of expeditionary ops the hierarchy have decided they want to move from the 'job for life', relatively intransigent Cold War RAF, in favour of a leaner model that reflects the Army a lot more, and this makes sense in many ways. As well as the benefits of a generally younger, fitter airman, promotion could become as meteoric as the Army, ultimately encouraging retention, while the thrusters who signed up since 2001 are under no illusion as to where they're going to spending a reasonable proportion of their duty time.
While the hierarchy harp on about it when it suits, one intangible loss in changing the RAF structure is experience, which in aviation is priceless. IMO a model somewhere between the two extremes would be desireable, where some experienced personnel are retained, while allowing young blood to get on with the job -
how to do that is the [£5500 - £15000 depending on specialisation] question.