It didnt sell well but people I know almost universally prefer it over the 15" Retina. They want real estate and ports. Now they have to buy a windows device to get the screen size.
I think with the advent of the 20 inch tablets like the Vaio Tap 20 (which is portable and useable as a laptop with its own special bag) I think there'll be a lot of people in creative industries wanting the big screen machines.
The 21 inch iMac is simply not portable - so Apple will continue to bleed the serious creative industry people who want portability and need the sweet spot of 17 - 21 inches of screen.
Trust me, once you use a 17 incher you wont go back to 15.