Well I have both a 2015 5k iMac and a 2008 24" sitting right next to each other on my desk. And you know what? They both look as 'thin' or as 'thick' as each other because I'm not looking past the screen at what's behind it. Thin on an iMac is just pandering to 3rd person aesthetics! These things could be a cube for all I care (actually, given past reputation, maybe not) - but even the thicker 2008 model offered 'some' extra upgradability beyond RAM, albeit in a 'don't do this at home' kind of way (such as remove the optical drive for an extra SSD). Going back to this form factor would not, in my opinion, be a backward step. You can only go 'so' thin before it starts to get ridiculous and that line has been crossed already. As it is, don't they measure the thickness of a MacBook Pro in Microns these days?