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.