Actually, there is a whole lot more to the cars being thought of as "disposable" than just the engine. A lot go by the wayside when the transmission starts slipping. Of course, most people have never changed the trans filter/fluid in the entire time they have owned the car, and will compain about the cost of a re-build. Also, smaller cars are generally built to be lighter, so use thinner metal on many parts that rusts through more quickly than a heavier car. Again, if people maintain (wash/wax/repair if needed) the body, it will last longer. Note: still not forever...when the frame rusts through, there are some safety issues to examine. Another one that is common, is that people with "cheaper" cars will tend to let little things go. And yes, I have seen it here as well. If the AC is out and it will be $400 to repair, they will let it go and just roll down the windows or remove the belt. Or the power windows go out and they will think it too costly to repair. If the care was not thought of as "disposable"....they would spend the money and time to fix it.
There is some financial sense to be had as well. My general rule is that when a single repair will exceed the book value of the car, it is time for it to go. Since I do 95% of my own wrenching, this is not too often, but it has happened in the past. When My metro got totaled, even if I had bought it back as salvage and chosen to repair the damage, I would have been upside down in the value of the car for more than 40K miles. The value was to look for something else....hence the neon. Was the metro "disposable"....in this case, it might be looked at that way.
Had my pickup sustained the same $$$ amount of damage, it would have been repaired since the cost would only have been a fraction of the value of the truck.
There is some common sense involved too
Jim