My comments were made with
GA, especially the light end
in mind.
We're now drifting towards the high end.
for IFR in CAS in Europe one needs BRNAV capability, for which the only means of compliance is INS or GPS.
Navigation is not my speciallity but IIRC B-RNAV requires 5 NM track keeping accuracy and compliance can be achieved by DME/DME and probably by VOR/DME as well. Do you know which AMC covers B-RNAV, I did only find the RNP-AR one.
Anyway that is not sole means GPS, but primary means. When GPS fails you can fall back on alternative systems that don't meet the requirements but at least give you a good sense of where you are.
GPS (with or without RAIM) is used as the sole means of navigation routinely (and 100% legally).
I doubt that, but again navigation is not my speciallity. GPS as primary means seems reasonable provided that you have RAIM (and you usually have it). But I don't believe that you don't have a clue where you are if GPS fails, at least I hope so.
ATCast