I don't find anything particularly wrong with galvanized cables or spokes. I used to build wheels with only Robergel double butted galvanized spokes. At the time, late '70's, stainless spokes were seen as not as durable. But there are downsides to galvanized steel, It will rust given enough incentive and the surface turns a dull grey.
With shifter cables, they should be greased when running through standard steel housing like was the being used when @
ejw bike was made. Between the grease and the galvanizing these cables always worked well. But we are talking about quality cable and the bike at least put in a garage or shed when not used.
For an upgrade, I think this option from Jagwire is a good one with the strength of steel and the slick outer surface. If you don't mind greasing the cable, it would be an improvement over the plain galvanized cable that was available at the time that many of our bikes were built.
Jagwire Slick galvanized shift cable
I have switched to stainless spokes, double butted still. I think of that as an esthetic decision. And it is tough to find Robergel double butted spokes nowadays.
Often, more expensive means it must be better, however, with cable, I don't know if galvanized cable, like from Jagwire above is inferior, it may even be better than stainless. Some of my homework indicates that it is. And no one will be impressed by you shiny cable like they will with your shiny spokes.