Although CEA is claimed at any one time by a relatively small percentage of personnel, there are many others who either intend to claim CEA in the future (and thus stay in), or who have claimed CEA in the past and are now past the 'golden' 35-45 age range where a second career is particularly attractive (and so stay in).
Secondly, although there is a current mood of 'make it cr@p enough for people and they'll leave, so saving us redundancy', this could backfire in spades. Although people may be unhappy, this doesn't mean they will leave - financial circumstances are unique to each family. You could just end up with a bunch of very hacked off middle ranking personnel.
Finally, CEA is there for a reason. It's not designed as a perk or a sbtle way to increase salary (contrary to the MPs' arrangements!). While personnel sign up to follow the flag in support of Her Majesty's business, they generally don't want their childrens' education to pay the price for such loyalty!