I thought that at least 2 of the units mentioned manufactured their own atco's from internal staff to cover for the contingency of retirements.
I know that there is still a shortage of fully qualified, experienced atcos, but when it really comes down to it, are there all that many atcos who would really leave their present unit?
Take into account the cost of moving (rarely met in full by the new employer), the personal/family upheaval, the pressure of new validations, starting at the bottom of the heap all over again and the distinct possibility of all these factors going the wrong way and not working out, I think that few would genuinely want to do so.
My own unit doesn't, by any stretch of the imagination, pay the top wage, but we have a very high quality of lifestyle, its a very busy but a good place to work, and all the staff are settled with only one or two younger folk who may move on, but only to enhance their long term career.
Nearly every atco I know has a twice yearly whinge about moving on to greener pastures, but they very rarely do anything else about it.