I'd definitely trust a modern solid state relay more (Triac, SCRs etc) those things will last for decades unless you pump twice the rated current and voltage through them. Of course that technology was not available back when. I'm surprised most of these aftermarket systems are using mechanical relays, most modern vehicles I'm sure use solid state parts as they must have dozens of "relays".
And look at modern wire connections, environmentally sealed and almost never fail or overheat ... but once again that type of product wasn't available to be sourced 50 years ago.
Probably unfair to compare old to new engineering because so much of it is controlled by the available materials, and the materials have changed tremendously. I'd say it's more often that our old cars electrical systems fail because of material degradation than design issues. Cracked insulation, degraded plastic connectors, lower temperature resistance, lower chemical resistance. In mechanical systems, worse gasket materials, poorer lubricants, no such thing as Finite Element Analysis, less exotic coatings.
I guess some of the more advanced stuff might have been available back then but I'm sure it was prohibitively expensive.
So far my last two GM truck engines have gone 100,000 miles with not a single major part, not a water pump, or alternator or ignition part (not even plugs), actually not even an air filter on either one. One did need a small evap valve. How much of that is better materials, how much is better design?
But then designers and engineers have software tools now that would have been inconceivable 50 years. They can probably simulate years of wear and tear on most component without ever actually making the part. They can model air intake systems for maximum flow, minimum sound and the least carrying capacity for particulates with a desktop computer. Same deal with exhaust systems (and of course now they are SS to comply with EPA requirements for longevity). I bet they can plug in the design for an entire electrical system and stress test it for hot spots and such long before it's ever made.
I guess in the final analysis, if the engineers were giving free reign we probably WOULD have flying cars by now, but companies have to make a profit and old ladies don't want flying cars
