I was under the impression that ground augmentation stations (for GPS) have a long range, and so don't need to be "near" the approach being flown.
Pity that MLS got killed off, really. It had some practical difficulties, and conversion was going to be expensive on the ground and in the aircraft, to put it mildly.
But it has huge benefits over GPS, mostly mentioned above in one way or another; accuracy and not being subject to deliberate degradation are two, and the more efficient use of airspace being another. Every time I see the straight line of landing lights approaching LHR for several miles I wonder why we still operate so inefficiently.