I've got 400+ 1:18 diescast in my collection, and have had very few problems from either the cheapest or most expensive. Regarding trains, cars or whatever, have we considered the way these are stored at our homes? I know some can display any and all of thier collection outside of the boxes, in a somewhat climate controlled area, but there are those who can't. Storage in attics, basements, in boxes with little to no air flow, repeated exposure to extreme temps, both high and low could and would absolutely accelerate any breakdown with our models. Case in point, and it didn't dawn on me until I read this thread, I met a guy who had an older MTH Dreyfuss, not sure on the manufacture year, but when I went to look at it, it was pristine. He explained that it had been stored in his attic since new, never removed from box until I opened it up. I had to pry the thing loose from the foam to begin with, and the outer protective foam wrapping had to be peeled from the engine and tender. No obvious flaws to the items themselves, but the close contact between the foam and metal in temps ranging from probably the 20's to the 120's and humidity levels in the 10% to 95% range.... I'm sure will have or could have some impact on the durability of the item itself.
I think we do this to ourselves. As a 1:18 collector, I buy sometimes because I want a certain model. Only to have it arrive and wonder where in the heck I'm going to put it, and it remains in its styrofoam only to be looked at on occassion. When I do get it out a few years later, there may be paint issues after having sat so long, metal or plastic bits falling off etc. I'll keep collecting whatever strikes my interest, can't have a fear of things happening years down the road. I could just as easily pick up a flawless engine from my layout and drop it on the floor and totally do it in altogether in 2 seconds. If it takes 15-20 years for some flaws to surface, so be it. Of course we all want to see our treasures last forever, and lord willing they may. Just my 2 cents.