They are well on their way but it will never be a direct requirement per say. The regulations state
(2) Two-way radio communication and navigation equipment suitable for
the route to be flown.
In many places a traditional VOR, DME, NDB or ADF may suffice as suitable navigation equipment however the FAA now has lower altitude GPS routes as well as many LPV approaches out there that are coming on line. Generally LPV approaches have lower minimums than their non precision radio beacon counterparts. Thus going into an airport that often sees weather may only be possible with an LPV capable (WAAS) GPS. In turn your operations may be severely limited without GPS moving forward.
The FAA will likely never institute a regulation that says something like all aircraft have to have IFR capable GPS' on board as thats not really how they regulate things. Eventually if they were to drop all ground based radio nav aids in order to be compliant with the above regulation everyone wishing to fly IFR would need to have an IFR legal GPS on board.
For the record "back country" airports are far more likely to have a GPS approach than a ground based radio approach as their may be no facility near by and GPS approaches can be simpler to maintain and add to an existing field.