FlyingforFun
It is indeed unlikely that GPS will be switched off or degraded by the Americans. Receivers are limited to 999 mph/kts to stop them being used by missiles and our transatlantic friends actually recently removed the deliberate error (SA was it called?) that was originally there to stop Mr Binladen.
You are right however to address risk. Because that what flying - and life - is all about.
The risk of unaided pilotage/map reading is you become lost. If you have ever been lost - I have - it is not easy to get un-lost. In the meantime you can look down and see a large airport. I looked down and saw a smallish one - fortunately I was above its control zone.
The other risk is you spend more time looking on the ground to locate yourself or gain reassurance. Using radio aids as the primary navigation method gives you more time to look out for other aircraft.
With GPS the greatest risk as far as I can discern is that people use map displays so they are not thinking about speed and track nor are they bothered about looking at a proper map. The displays do not show controlled airspace reliably (nothing to do with GPS of course) so people bowl into a zone and give this method of navigation a bad name.