That's definitely not the case. GPS, RNAV, and little magenta lines aren't necessary to operate in the enroute environment. They're nice toys, but they definitely aren't necessary.
One day, Guppy, we will all work out which part of the known universe you fly in, IFR, enroute, Eurocontrol airspace.
It's not
Europe, for sure.
Without BRNAV you are illegal anyway, in any practical flight. Personally I wouldn't write about doing it, just in case you get rumbled one day.