Call me a bit dim here if you will, and I’m not a chip designer, but if they are working within the temp range they have been designed to, and throttle themselves accordingly, and worst case I have AppleCare, what’s the worry here apart from a bit of fan noise? I really don’t understand why someone would get rid of the machine unless they need a workstation that will run CPU intense stuff 24x7 in which case you need a workstation class Xeon process i.e. iMacPro... Is there a lot of factual evidence and studies to say these CPUs burn out? Just asking...
Generally speaking, heat is the enemy of electronics. Intel and other semiconductor manufacturers specify the maximum safe operating temperature of a device. For CPUs, when that temp is reached, the chip will automatically throttle back its clock speed to reduce the temp. If it didn't, the chip would be irreparably damaged. (I think the thermal design may factor in the length of time the chip is operating at the maximum temp before throttling back.)
For a CPU, the maximum operating temp is designated as the T-junction max temp, which is measured at the die -- not the case that the silicon chip (the die) is packaged in. For the 7700K, that temp is 100 deg. C. The CPU clock speed varies according to workload, and so does the temp. Constantly operating the chip at its maximum temp over long periods of time might cause unwanted effects at the microscopic level (e.g., electromigration as an example) which could decrease the life of the chip. Modern chip design tries to take these effects into account to increase reliability. And it's possible that any failure might not occur during the useful life of the chip anyway, i.e., the computer gets replaced for something newer before the chip might fail.
No two chips are identical even though they're the same model and manufactured the same way. (If you look at the microscopic features of a semiconductor under an electron microscope, you'll see what I mean.) So one might fail where another might not, given the same conditions.
But the CPU isn't the only component that gets hot. The heat generated by the CPU causes other components in the computer case to operate at higher temps. And those components could have their life shortened by higher heat. Also, the very thin metal conductors on the printed circuit board will expand and contract with heat cycling that could cause mechanical failure. The iMac motherboard is in a fairly confined space with only one fan to exhaust the heat. (The iMac Pro has a redesigned cooling system with two fans.)
Apple has lots of smart engineers. They've taken all of this into account in designing the iMac that incorporates the 7700K chip, (which runs fairly hot even in PCs that have bigger cases with more fans, better air flow and/or liquid cooling). In my opinion, the odds are quite good that the chip and motherboard components won't fail due to heat during the useful life of the iMac. But a very small percentage might experience heat-related failures where a cooler running chip, like the 7600K, might not.
When I bought my 2017 iMac, I had a hard time deciding between those two chips. Ultimately, I decided on the 7600K because I wanted a quieter computer, and my usage didn't really require hyperthreading and the slightly higher clock speed of the 7700K. I rarely ever hear the fan in my iMac.
