Future of GPS
I am providing a very brief response, and one that does not answer fully all points (lack of time on my behalf).
GPS & Glonass currently make up GNSS (Global Navigation Satellite System ) and Galileo will join this mix. All three together shall provide a reasonably robust mix.
Areas of weakness are physical attacks to ground based and space based components along with jamming of signals. Another way would be the introduction of spurious data signals.
Currently the GPS (US system) utilises L1, L2, L3, P1 and a number of other frequencies which are or may come on line as required. GLONASS and Galileo use other frequencies. You have multiple frequencies for each system and multiple point sources as well. All of the systems depend upon a ground component to give increased accuracy. If any one tries to Jam GPS systems it is very, very difficult. This is due to multiple redundcies of frequencies, satelittes, software, etc.
As for physically attacking satellites that is currently extremely difficult and extremely costly. In the future when laser technology may be more effective at a lower cost satellites can be protected. Costly, but it is possible. Satellite missile attacks are more difficult to protect from.
There is no techology currenlty available which provides the seeming simplicity and accuracy of GPS systems (used here as a generic term for satellite based positioning systems).
The wide spread usage within civilian systems (from mobile telephones to surveying and from vehicle trackers, to oil exploration) tends to provide some system protection in that it is unlikely that systems will be withdrawn from use.
Hval
Edited for an incorrectly placed full stop