Nice idea but it will never work.
I remember a lecture way back about the Royal Navy. During it, the subject of manning came up and the presenter ( a 1* IIRC ) stated that one of the biggest mistakes the RN made was to halt all recruiting in the mid 1990's. The effect was to create a demographic / rank " back hole " which was working it's way through the system and its negative effects would be felt for 20 odd years. I stand by to be corrected by the Navy types
Reduce, yes - but to stop is not a good idea as we will always need young blood. Sounds like a bitter pill to swallow. As for the x - over you still have the problem of X chasing Y slots and you have to open the competition up to all, otherwise it's an HR train wreck and the D word will get wheeled out very quickly.