Unless they disable Turbo Boost (which is horrible for performance, but great if you want benchmark consistency), the CPU will automatically overclock until it reaches the limits, adjusting both voltage and frequency.

All the evidence I've seen points to electromigration as a cause of this degradation, and IMHO excessively aggressive automatic overvolting by Intel's microcode is to blame.

There is actually a simple experiment which can determine whether that is true --- remove the fan from the heatsink, or even let the CPU run without a heatsink. As the CPU will automatically throttle once it reaches its designed maximum temperature (and AFAIK that is a hardcoded limit), it will lower its frequency and voltage to maintain that temperature. If this results in a stable CPU, while the one that has great cooling becomes more unstable, it confirms the hypothesis.

There are numerous stories of machines where the heatsink was not in contact with the CPU for some reason, yet they remained perfectly stable (but slow) for many years. I can also say that I've had an 8th-gen i7 running at 100% 24x7 with all power and turbo limits disabled, with its temperature constantly at the design limit of 100C, and it has also remained stable for over 5 years.