GPS jamming is already thought to have caused one drone accident, with fatality: http://lemondronor.com/blog/index.php/2013/3/gps-loss-kicked...

And there have been several close calls already, with passenger jets: https://spectrum.ieee.org/faa-files-reveal-a-surprising-thre...

Planes don't need radar, transponders, or even radio to fly, but they're all very important for safety.

Yeah exactly, like that second article says, suddenly having to switch to a pretty archaic system that's now only used for backup (with a reduced number of VORs) is not fun, and the distraction can lead to build up of other situations that can really cause serious danger. It won't be the single cause of a crash, but crashes are rarely the cause of a single problem but rather a compound failure of many things going wrong in just the worst way.

Here in Europe VORs are also very rare now. Makes sense because they're hugely expensive to operate and when used only for backup it's not a very good investment.

We're actually replacing a lot of VORs for GPS nav and GPS approaches are also a thing.