GPS is a DoD run system that has evolved into a "utility" that everyone uses. It is still a DoD system, run by DoD for DoD. The rest of us use it at DoD's pleasure, and they can turn it off at any time. This is why other countries have / are spending a lot of $$ on their own GNSS - they don't want to be subject to DoD's operational restrictions on the system.
GPS is a wonderful system - it increases positional and situational awareness, allows direct routings IFR that save thousands of barrels of fuel daily and saves lots of lives. But, it is a military system, so don't be surprised when the military turns it off or degrades it.
Remember that SA (selective availability) was turned off in 2000, but DoD says that they now have other means to degrade or deny GPS signals to anyone, anywhere, anytime.
So, this points out the value of the lowly, old, low-tech VOR - the FAA runs them. I'm not sure if this is good or bad, but it's unlikely that the FAA will decide to turn them off at any time, at least not on purpose. There have been other threads about equipping an airplane for IFR with GPS only; hopefully this points out that it's not a great idea.
And yes, what does that mean for ADS-B? Since the FAA has committed to basing everything off ADS-B, which is itself based on a GPS signal, the entire air traffic system will be dependent on good GPS signals. Indeed, this needs some more consideration.
TODR