You are seriously unlucky and a rare case. Out of about 30 Macs I owned only the one with the famous faulty Nvidia GPU needed a logic board replacement.
Yes, I would have to be a rare case, otherwise Apple would go out of business. [Though I'm not sure if it was purely bad luck*.]
Nevertheless, the point of sharing my anecdotal experience was to suggest that computers aren't necessarily less reliable now than in the past, and that to the extent one can find a reliability differential, laptop vs desktop may be more likely than new vs. old.
Of course, what we really need to addres this is data, not my anecdotes.
*It always puzzled me why this repeatedly befell me, since I thought I babied my laptops--they spend 90% of the time in the apartment, propped up on an Rain Design M-Stand which provides good air circulation and puts them above any spills.
Then I realized that, precisely because I was using them mostly in desktop mode, I ironically was probably actually being harder on them than the average user because: (A) I was always driving an external monitor, which meant the dGPU was running continuously; and (B) while well within Apple's operating specs, my apartment was typically on the warm side (low-mid 80's) about 9 months out of the year, because for many years I used window fans rather than A/C (and I'm on the top floor). Consistent with this, other than the faulty NVIDIA GPU issue, and one swollen battery, the failures I got were that my laptop would begin to thermally throttle (i.e., after a few years of use, I lost my ability to run them in the low-mid 80's with an external monitor attached) (kernel task >~400%). In sum, while I was using the laptops well within Apple's specifications, I was probably thermally stressing them more than the average user.
N.B.: The thermal failures were for the 2011 and 2014 models, not the 2008. And I recall one of them had to have its logic board replaced twice for this reason.
Unfortunately, non-engineers tend to have an unhealthy fixation with temperature. We have to deal with an entire generation of PC users that religiously believe that their computer will turn into dust once the CPU goes over 90C. Go figure.
While I don't think my computer will turn to dust (in spite of its unhealthy fixation on human blood, and seeming aversion to sunlight), I hope the above anecdotes explain why I have a less sanguine view of Apple's thermal design decisions than you do.
[Also, you never responded to the TDP numbers I posted about the current large iMac, showing that its original processors (those in the introductory model) would have created high fan noise with their designed thermal solution which, again, I find unacceptable in a desktop given there is a relatively easy fix:
https://forums.macrumors.com/thread...or.2307038/page-6?post=30436381#post-30436381]
