Making them thinner would indicate to me that they aren't planning on making them any more expandable - though I would happily accept being wrong about that. My whole issue with iMac is that they aren't expandable enough, at least not if you want to keep your warranty.
YT
I don't see how 2 inches is thinner as I just measured my 20 inch iMac width and it currently measures 1.5 inches. Not that it matters to me but 2 inchs is not thinner in my iBook--)))