To say GPS can't be used because it might be switched off or degraded is not a valid argument. ILS or VOR can become unavailable.
The problem with GPS is that it offers no consistent, guaranteed service. It is perfect on 9x% of occasions, quite good on 9y% but not reliable in 100% of situations. This means you have to have Plan B for the 0.000z% of occasions, and if you have a Plan B you might as well use that all the time.
NASA or FAA have analysed this in great depth and if I remember correctly they think several new things will have to be added to permit primary en-route use.