I think his response is perfectly reasonable. There is always going to be a compromise between Cool, Quiet, and Life Expectancy in a PC.
He can't really say much about the Firefox/Amazon Assistant. There are always going to be bits of code that don't play nice, and that is just an example. That's why HTPCs and Servers should run minimal "extra" stuff.
My workstation, where all sorts of stuff is installed and high load stuff is run, has a keyboard with a CPU and RAM load display on it. So I can see immediately if something is going crazy, and fix it. On an unattended HTPC, if I had concerns, I would install a temperature monitor that would record a temperature history, and could send me alerts if there was a problem. Just like HD Sentinel does for my hard drives. I'm sure there are such beasts around. Maybe there is one that would even force a shutdown at high enough temperatures.
In terms of life expectancy, how long do you want the PC/CPU to last? I have my workstation set to panic if the temperature goes over 70°C (from memory, T
CASE MAX is 67.9°C), and it rarely does because it has variable Noctua fans in it, which run up whenever the temperature rises. That CPU is now seven years old, and I expect it to last another three, if I want it to. If you were hitting the level of thermal throttling every now and then, I would still expect five years out of a modern CPU, unless you were very unlucky, or the cooler wasn't seated properly. But that would be noticed quickly.
Your alternative is to install a fan controller that doesn't turn the fans on until, say, 70°C, then ramps up from there. I like that approach, as it give me peace of mind if ever something does hog the CPU, or I'm working on a hot day and room temperatures get high. Also, maybe your motherboard and BIOS is capable of turning off fans and only starting them at higher temps. I think some are. I use a
SilverStone 1-to-8 PWM Fan Hub to drive four 80mm and two 120mm fans off the one properly motherboard controlled CPU fan header in my workstation. My motherboard can't turn off the CPU fan, but it can run it pretty slow at low temperatures, so the PC is mostly pretty quiet.
Completely fanless is a great aim, but I reckon "No fans running, most of the time" is a better solution in the long run.