Wild and unfounded speculation: I feel that the thinness of the new iMac was not an explicit design exercise, but rather a consequence of bringing the M1 over to the desktop. Because of how powerful the M1 is, they were just able to recycle the already thin laptop designs, putting them in a desktop enclosure. They couldn't do this before, because Intel mobile CPUs didn't match the performance of their desktop equivalents. And since those desktop CPUs had more demanding thermals, they couldn't make a thin iMac.