Back to the good old days, VAR, VOR, DME, LLZ, ILS and ADF.
Maybe if GPS was designed from the beginning as a navigation system, we would not be having this discussion about jamming.
Same with the WWW.
Using both for things that were never envisaged.
GPS will always be highly susceptible to jamming. It’s a very low power system by design. To come up with a more jam resistant system would require far more power out which is simply not possibly with current of envisaged future technology in satellites on the scale needed to provide worldwide coverage.