There are currently only 40 ILS frequencies. The directional nature of the beams and relatively low power mean there is not much interference. However that's not to say it doesn't happen. A few more would always help.
Contrary to certain opinion GPS alone does not meet the accuracy requirements for low vis approach (I think we're currently at RNP 0.1 which is about 15 meters. ILS accuracy is the order of one to two meters at touchdown). In order to do so, at least one (thought to be 2 or more) ground based devices need to be located at extremely accurately defined positions, to transmit to the aircraft the local error in the GPS signal, so the aircraft can accurately fly the vertical/horizontal profile (called differential GPS)
ILS is old, established and certified. DGPS is not.