They could be fixed using low-tech tools though.
Something you're overlooking with your concept of "reliability" is that cars produced before 1980 can be fixed using pretty basic tool. It's conceivable that a car from that era could continue to work well into the 2100s. Because most of what can break on can be remanufactured by a reasonably-skilled machinist.
Yeah, cars need to be fixed much less often, and they are substantially less fickle vis-a-vis temperature and operating conditions. But the moment the software to repair newer cars is gone, so too goes our ability to easily fix them.
This isn't some far-flung dystopian idea either. It's totally possible that the hardware & software used to repair today's cars is difficult or impossible to come by in 30 years. Some of this stuff is five- and six-figure investments. Maybe stuff for GM or Honda vehicles will be accessible, but what about smaller companies like Mazda or Subaru?