I don't think this narrative quite fits with the reality of how Apple creates products, or what we've seen recently. Honestly looking back at product announcements Steve himself seems pretty keen on thinner, he usually lingers on it a moment... also recent iPads aren't thicker? The new Pros are exactly the same as the 2018s, the 10.2 is exactly the same as the 9.7" iPads before it, and the mini and Air are the same as their immediate predecessors. iPhones have been getting steadily thicker again for years after reaching their nadir with the iPhone 6.It's pretty clear that Jony Ive was running the design show as much under Tim as he was under Steve. Steve might have been able to tell Jonny "no" more than Tim was, but Ive's (presumed) obsession with "always thinner" was as strong under Steve as it was with Tim and this is why we got things like the butterfly keyboard and iMacs and MacBooks that could not handle the heat Intel's busted-arse CPUs were putting out.
Now that Ive is gone, the MacBooks are getting thicker (as are the switches under the keyboards), the iPads are thicker, the iPhones are rumored to get thicker and maybe the iMac will also get thicker so it can cool those hot CPUs better and Apple can put in a half-way decent webcam.
With regard to the stagnating of iMac design, I can only really suggest between 1998 and 2009 Mac desktops probably weren't less than 20% of mac sales as they are today...