The biggest problem was with Detroit engineering design dictated by the bean counters that minimized vehicle durability (and hence (real economic)value and residual (perceptual) value) especially compared to Toyota.
Truck margins in the late 60's early 70's were low compared to cars. After the gov't introduced CAFE and we got the Mustang II, most folks began to realize that trucks and truck based station wagons (Suburban etc.) represented value and the margin situation reversed.
Then Congress conspired to increase oil prices and then tanked the financial markets by promoting subprime real estate loans and voila, here we are.
I think you make a good point that gets almost no play when discussing Detroit's problems. Certainly, the Unions and the Government shoulder much blame (and deservedly so), but management made some serious business model mistakes going back decades.
Certain Japanese, German and probably British cars retain more of their resale value because they're better engineered and better built with higher quality parts and materials. There's just no getting around it.
I guess the only question to ask, is... Did the Big 3 merely respond to what the American consumer demand - cheaper more expendable cars, or did they create the demand for the consumer? I just don't know which it is.
Entry level cars became pregnant roller skates, and the foreign manufacturers who had been building smaller cars for years dominated first that market, then moved into larger vehicles.
For those who were paying attention, Japanese motorcycle manufacturers pulled a similar coup, starting with small displacement motorcycles and working their way into the big bikes. While Harley-Davidson survived, and after change in ownership, management, and design, (while riding solid brand loyalty) flourished in the big bike market, there are more Japanese brands (even though some of those bikes are built here) in the US than American marques.
Auto manufacturers did not evoke the same fierce brand loyalty after their entry-level cars were ill designed and of generally questionable quality by comparison to their foreign competitors, and those who moved up were perfectly willing to look at the next level of imported vehicle, looking for value over anything else.
While no one car buried Detroit, the CAFE standards contributed, and buried a host of motorists since their inception as well.
Can we put at least part of the blame on Congress where it really belongs?