My father-in-law was a mechanic also. He would not disagree that the bulk of his work was easier. (Because most of it consisted of the much larger amounts of maintenance older cars required.) But he also does not have any starry-eyed notions about cars from that time being more reliable. They required a lot more work for miles driven, which is exactly what you would expect with cars that headed to the scrapyard after 100k-ish. (Counting the number of years a car "lasts" without considering how much it's driven, is pointless.)
And there are plenty of issues with older cars that require tinkering; intermittent running problems under load are downright painful to diagnose without an OBD system.
As you said if you do proper maintenance any car should last forever. But the cost of maintenance increases exponentially when there are significantly more components in an engine compared to a simpler engine. But that is the price you have to pay to have all the luxury of modern era cars.
I don't know how I can put this any simpler: This is ONLY true if the quality of the parts and design cannot increase to keep up. But that is absolutely not the case. If it were, the useful life and reliability of a car would have decreased dramatically as they featured more parts. Instead it is utterly routine for a car to go to 200-300k with nothing more than cheap wear parts and scheduled maintenance. The idea of a 1960's passenger car engine routinely (with no special care) lasting for 300k without a complete engine tear-down and rebuild (possibly more than one of them) would have been astounding. Everything on a modern car lasts longer: tires, paint, belts, hoses, engines, alternators, batteries, interior parts, electrical bits, fuel pumps, radiators, transmissions, ignition parts, clutches, brakes, sheet metal, fluids, EVERYTHING, except, apparently, nostalgia.
"More reliable" cars would not have been scrapped after 100k or so. (Barring body damage, few junked cars today have such puny mileage.)
P.S. No, the shuttle was not prohibited from going to the moon because of reliability concerns. Because of the size of the payloads it was designed to (and did) carry from the start, the idea of building a rocket that could transport enough fuel to get the shuttle, and a payload, to the moon and back was (and is) unfeasible. The Saturn V booster was designed for a 45T payload. "Dry", the shuttle + payload weighed 115T. (Even without payload, the shuttle was 82T) It was designed from the start to "shuttle" loads between earth and orbit and was never intended to carry enough fuel for a lunar trip. It could not also handle re-entry from a moon trip without a retro burn (requiring even MORE fuel.)