It's only progress of technology if you can make it thinner without limiting/sacrificing performance, features, reliability, and repairability.
Each generation has been faster and offered better performance. I doubt the latest iMac would offer any less.
For the most part, comparing the successor of each or the equivalent of previous generation to current generation:
AMD 6000 series cards < Nvidia 600 series
Sandy Bridge < Ivy Bridge
All while consuming less power. Sounds like progress to me
Throw SSD/Flash into the mix and we got ourselves something faster and more reliable (Seagate Hard Drive replacement program anyone?)
Additionally, the 2009-2011 model iMacs were thicker to accommodate the 3.5" disk.
Seems like the 2.5" disk will be responsible for the reduction in thickness.