Modern cars are getting harder to repair (smaller spaces, closer quarters) than in the past.
I also think the entertainment electronics are getting much more tightly integrated. When I was in high school 15 years ago it was pretty common to replace your stereo (and get a CD player!); but now the whole system is so integrated into the look and feel of the car anything aftermarket will just look out of place.
Upgrading an interior probably has too much diverse meaning to meaning (does it mean dashboard? seats, console? etc), so let's leave that one for now, but I think the rest of the issues you mention have possibly already been addressed.
Older parts don't have to be more expensive, because we'll be able to 3-d print metals, or assuming we're talking about an electric car, there are less mechanical parts anyway and a company could decide on a modular design that they stick to for efficiency purposes. For instance, Toyota could decide on a shape and size of their batter pack. Then as battery technology inproves, they don't have to create a bunch of stock to upgrade old cars, as the old cars would use the same design as the new.
I agree with you regarding smaller spaces and closer quarters, but technology is also constantly shrinking too, so I think it might be possible that replacement/upgrade parts could be smaller than the one they are replacing, so as long as they share a mounting structure, it won't cause a problem.
You are right about entertainment systems, previously there was a standard (I think it was called DIN) size and there was only one size. Then they added 1.5x and I think 2x sizing. But upgrading your entertainment system used to mean getting an aftermarket system, not a system from the same manufacturer. Now, it seems to me, that manufacturers use the same or a very similar system across their brands. They aren't stocking 100s of different systems, they are stocking 5 or 6 and those are used across all of their designs.
If upgradability is thought important from the design stage, I think all of this is possible, and more.
I can see it having difficulty operating on small or even medium sized purchases, but a car is a very different thing. Why make a large purchase when you don't have to. People already know new cars loose so much value the second they are driven off the lot, why would you subject yourself to that if you can upgrade your existing car.
From an environmental standpoint, isn't upgrading an existing car much more friendly to the environment that digging up new metals.
As far as things like the interior wear, why not be able to upgrade a new interior. Why trash an entire car just because part of the interior is worn. So you need a new headliner, get just that. You don't need to throw away the whole car.
I'd agree that large scale changes like completely autonomous control may be difficult to retrofit, but probably not impossible as many cars are being built with park assist which means autonomous control could be mostly a software change rather than hardware. At the same time, how often do we see massive technology changes like that in the automotive space. More airbags could be tacked on to older cars, brakes upgraded to ABS could have happened. Basically, anything that has happened in the automobile in the last 50 years could probably have been a minor hardware upgrade. Even carburator to fuel injection could be an upgrade.
I also think we're experiencing a cultural shift in why and how people buy. I think the idea of just buying something new, is becoming an outdated. I know I don't want to buy just to buy, I don't go "shopping", but I definitely did in my younger days.
Lastly, can you please not state as fact what is clearly your opinion. To say "No it won't" is a very abrupt and I'd say rude response and seems to attempt to shut down discussion rather than saying "I don't believe it will have implications because..."