The "designed failure" is a nice conspiracy theory, but I really don't think it's true.
What I think is true is that given the constraints allowed to the engineers when building a vehicle has more to do with how and when parts fail. After all, government regulations continue to tighten their requirements for safety, fuel efficiency, and emissions. All the while, consumers want to have all of the latest technology...at a "fair" price. The government constraints adds complexity to the vehicle, while the consumer needs compromise superior build. After all, if we had "aircraft-quality" components in our Mercedes, they would probably sticker for about what a small airplane would go for.
Metal parts themselves do not break down, rather the moving components wear out beyond tolerances that render the part ineffective. The rubber seals, plastic wiring insulation, and such are very prone to chemical breakdown, whether it be the fluids that they come in contact with regularly, or the temperatures they reside in, especially fluctuating temperatures.
As far as failure due to design, it's impossible to simulate the myriad conditions that the thousands of parts contained in a car may encounter during its lifetime, given the short lead time given to engineers when developing a new product.
I have a TV set that hasn't had a problem since I purchased it new in 1984, but then again, it doesn't have to contain seven types of volatile recirculating fluids, heat up to internal temperatures of 500 degrees and then cool down, and be able to withstand external temps of -40 to 115 and maintain a glossy finish. Plus, the chassis doesn't have to withstand the structural stress of 120+ mph speeds. How many TVs could survive a 5 mph impact, much less a drop from the lower shelf?
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car