I believe it's impossible to always know why a part fails and goes south. Much of it can be attributed to unidentifiable lower grade OEM replacement parts, versus brand new parts that were used to manufacture the car. I also believe that upline and downline parts degradation is very real, and a common reason for multiple, domino effect parts failure rates. That is, you replace one part or component with a stronger part that causes failure in interfacing parts or components. There is also another reason why things fail other than overuse, and that is underuse. Mechanical things do not work as well when they are not used on a routine basis - whatever routine is to you.

As the original and current owner of a 83 300D, I can respond with some authority.

I'm on my 4th AC compressor. The first one lasted 22 years. I only bought new (not rebuilt). One only lasted a month but it was covered by warranty. The others seemed to fail just past the warranty period.

The original heater control valve lasted at least 15 years. That's now become almost an annual repair.

Transmission was rebuilt about 90k miles. Rebuild is showing some problems but it lasted longer than the original parts.

All of the other parts are original or first replacement.

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/showthread.php?t=296386

This is what drives some of us at times to buy a new or slightly used car - especially when we travel out of our cars to make a living, versus going to and from a crosstown workplace. When a car is an extra car, or hobby, then there's no issue with dependability. OEM, I am finding does not mean new, with all parts. As discussed in my last post, #16, upline and downline parts failures exist too.