Indeed. My board has not seen high temperatures. I don't know what uC they are using but really can you deliberately make such poor quality chips on purpose?
In a perfect world, where handling and installing uC on the PCB is done with perfect precision and care, the uC would live very, very long. However, there could have been a micro-fracture undetected in the silicon of this single chip and installed and constant cycling and stress from handling and installation could have made it worse. Even though the PCB was not in a very hot environment, as you say, the PCB has some movement and heat fluctuations over time and this could have further stressed the chip until finally it did not function.
Also, as you pointed out, the capacitors look new and clean, however, I have seen very clean and new looking caps have very low ESR over time, which diminishes the filtering ability. Also, over time, the capacitance diminishes, therefore diminishing the filtering ability. The uC has a tolerance that allows voltage pops and the caps provide this protection, until it doesn't and the uC can take no more and breaks.
There are 10s if not hundreds of other scenarios in which this uC could have failed, mostly related to handling, electrical, installation and environmental stress conditions.
When you think of all of the conditions in which you have a perfect PCB, the probability of meeting all of the perfect conditions is so astronomically large that the devices will not eventually fail that it is what I would call a success that you got 9 years of service from this system.
Just my 2 cents...