It's not always that when CPU dies, the motherboard dies, but i guess it can happen. Usually the opposite happens. If the motherboard dies, especially with a mosfet failure, excessive current passes to the CPU and the CPU is fried. But this doesn't always happen either.
At any case, all electronic components have a MTBF. I was still a teenager when i was reading in computer magazines the old motto "for 10C of heat reduction, hardware doubles its life". This is a page i took at random from a PSU and it's rated at 25C. It is interesting to see that even the most lowly part has a MTBF at a certain temperature. When these are forced to operate in higher temperature, their MTBF changes for the worse. So, there is one breaking point in absolute limits (a sudden spike too high) and there is another breaking point, often ignored, which is slow, but inevitable in time and also suffers from higher temperatures. Even the PCB itself, has a MTBF... This is what i mean by not stressing something to its limits. That something can survive for brief periods of time a certain temperature, doesn't mean it will always survive it. On the contrary, it means, it will shorten its lifespan over time. It's like with capacitors. Capacitors are usually rated for 105C. If you operate them at 85C, it doesn't mean they will never fail. It just means they won't fail so soon as in 105C.