The irony is that when controllers leave, they take with them a lot of local knowledge and experience that simply can't be replicated in a few months training. New staff bring new dynamics to a unit, not always positive! New staff may arrive from smaller units and use their new unit purely as a stepping stone to better things therefore they have no vested interest in their new surroundings, their OJTIs are well aware of this and therefore resent putting a lot of training effort into someone who has no intention of staying. Morale continues to decrease amongst the original staff, perhaps more retire or leave - the cycle continues until you gave a very unhappy and unsettled place to work. Its sad but I have seen it happen.
I agree with Coffee above - if management ignore the human interface within a unit then they will suffer the consequences at some point, particularly as the market is so positive for ATCOs at this time. I don't know what the current pay and terms and conditions are at Newcastle but I do recall in years past it always had a relatively stable staffing level and seemed to be a good place to work. Fascinating how a few changes can have such a major impact on a unit!