For what it's worth ..............
In my opinion. I always thought it was about command; the whole point of the RAF is to fly things, the commanders at all levels in flying squadrons need to be aviators of some sort. To develop the best commanders, you need a large pool of potential candidates, if you introduce Sgt Pilots into it, you would be diluting that pool.
As others have already said, there is no clear financial saving, so what benefits would be gained?