Everything you say is very true. Why should engines be built to last longer? Most of the American public still thinks that a 100,000 mile vehicle is junk, and they're just about right. Unless it has been babied, the door hinges, seats, dash, paint, etc. are trash, and when they feel that the car is shot they won't take care of the rest of it even if the mechanical parts would be OK with good maintenance. Anyway, we're in a disposable culture...cars included.
And, the engines are built for low emissions and high fuel mileage without equal consideration given to durability. The high placed piston rings minimize the pocket around the piston crown where unburned hydrocarbons can hide out and increase the emissions, but as you said so well, also get carbon build up in the ring grooves. The short piston skirt gives a lighter piston and less friction for a sliver better fuel mileage, but severe cold start knock (piston slap) may result with cylinder bore wear. Again, little or no consideration for engine durability.
I'm trying for 300 or 400k from my cars--best lubes, maintenance on schedule or earlier if needed, cars in the garage instead of cars in the sun and junk I never use filling the garage. We'll see...time will tell.
Ken