high temperatures ARE a risk to the lifespan of the machine. frequent changes in temperature can cause solder to crack and detach chips from the main logic board. this isnt speculation, this has happened several times through history, the Apple III, the Xbox 360, the g4 ibook and probably a ton of various pc models that arent noteworthy enough to be remembered by history. a more recent case of this happening, the gpu failures of the 2011-2013 Macbook pro.
http://www.macworld.co.uk/news/mac/...es-petition-lawsuit-repair-programme-3497935/ note that almost every single comment on the article reports having experienced the issue
Apple had to set up a repair program after a 12,100 posts and 4 million views on a support community thread complaining about the issue
https://discussions.apple.com/thread/4766577?start=30&tstart=0 as well as a 38,000 signature petition
https://www.apple.com/support/macbookpro-videoissues/
as for fried retina imacs. ive seen at least one possible case here on macrumors
https://forums.macrumors.com/thread...ts-and-crashed-after-using-photoshop.1968019/
chip failures due to overheating take a while to show up because the solder needs to be heated and cooled down several times before it cracks. it can take a few years for the damage to show. people have all the rights in the world to be worried about the 5K iMac suffering a similar fate as the 2011-2013 Macbook pro