The Instrument Landing System was conceived in 1938 by Sperry and Reed Pigman (jointly) and has proven to be very precise over the years, and is a known quantity...very important for aircraft navigation.
GPS on the other hand, does indeed have its limitations, and especially so with precision approaches.
The wide area augmentation system is being implemented in the USA now, and indeed one can equip the aircraft today with this kit.
WAAS provides VNAV for many GPS approaches, thus allowing lower minimums.
LAAS will provide landing minima to corresponding CATI ILS minima if trials now in progress are successful.
Now, many outside the USA generaly tend to throw verbal stones at GPS, and the simple reason is...they suffer from the 'not invented here' syndrome, and of course that is their problem...and they are welcome to it.
Stand alone GPS sets, however successful they have proven over the years, need to be treated with a good deal of respect, just as you might with a modern FMS, and the old saying 'garbage in-garbage out' is just as applicable...they require careful detailed operator programmimg to function properly and reliably.
In short, don't expect ILS to disappear anytime soon.