Ah, sorry, I have overlooked it. I mean, I can't speak for Apple, but the simplest explanations that they didn't feel like doing a major internal redesign for a model they knew is approaching end of life anyway. The 27" iMac was released around 2015 if I remember correctly and it's thermals very absolutely ok. I can definitely imagine that they are getting more constrained with the hot running Comet Lake CPUs, but frankly, should Apple do a redesign every year because Intel can't keep it in their pants... and I mean thermal profile?