Why don't they just invest in GPS/RNAV approaches through EGNOS?
Errrrrr -probably because it won't help! RNAV Approaches (for now and the near future at least) have HIGHER minima than a standard Cat I ILS - in other words ILS will get you down lower/closer than a RNAV approach before you have to go-around.
In the medium term I am sure this will change but nothing changes in a hurry over critical equipment/procedures like that.
Most current RNAV 'installations' are to replace Non-Precision Approaches like VOR and NDB ones.
DD