A better software analogy would be calling an API method whose signature has never changed over 20 years. It would be like making a call to the Windows API, one that has been available since DirectX 2.0a, to play a sound clip.
Styling and design still have room to work if your engine compartment standard does little more than define a minimum volume of unobstructed interior space and several fixed mount point locations. You can make an ATX computer case look like practically anything, so long as the screw holes are in the right place, the I/O shield is the right size, and you leave enough internal clearance for the CPU cooler and video card.
The mount point locations became standard on outboard motors in the 80s, when Yamaha decided to use the same pattern as BRP (then OMC), who had in turn copied the pattern used by Evinrude in the 60s. So you could possibly buy a 50-year-old boat and mount a new outboard motor onto it without drilling any new holes or replacing the transom. The outboard motor bolt hole pattern standard does not affect the thickness of the seat cushions, or the size of the docking cleats, or anything else. You can have many different standards that are not mutually incompatible.
The idea is that auto manufacturers would add to a list of standards, rarely obsoleting some. Maybe the 20-year-old car only conforms to the engine compartment standard and the front-wheel-drive automatic transmission standard. The 15-year-old car conforms to those and to the steering wheel (with airbag) standard. The 10-year-old car conforms to those, plus a side-impact standard and a rollover safety standard. So you might not be able to upgrade the 20-year-old car to be as safe as the new car, but you could certainly upgrade it to be as fuel-efficient or as powerful, which might be all you really care about until the frame rusts out. If you are lucky, the old car might have been the basis for a newer standard, and you could upgrade some things as though it were conformant to begin with. A new car would certainly conform to all the existing standards, and if its standards-conforming side-impact air bags were recalled 3 years later, any mechanic would be able to replace them, instead of just the dealers for one maker.
Any given standard would only be replaced by an improved standard, and that might still be backwards compatible to the old standard. So I certainly would take that future-proofed 20-year-old car with a brand-new off-the-shelf powertrain in it, especially if it cost only $8k instead of the $24k I might otherwise have to spend on an entirely new vehicle.