Exactly. So the "best" setting on a 27" Retina iMac (4k) would be a practical workspace of 1920x1080. Granted, this is less work area than the current 2560x1440, but it will look amazing. And you can always bump up to 2560x1440 or even 3840x2160, just like you can increase the work area on the rMBP. I jump between the different options all the time on the rMBP and they all look great. It seems like the best solution and the most cost-effective one (vs. going 5k).
Remember this is exactly what Apple did with the rMBP. The regular MBP had a workspace of 1680 x 1050, but the "best for Retina" setting on the new rMBP was actually a downsize of 1440 x 900.
Apple may very well be testing a 5k iMac, but for next week I'd put my money on a 4k iMac in the $2500-$3000 range.
I'm tired explain that pixel doubling it's just an lazy retina implementation, as long the scree it's retina OS programmers can write guí elements targeting an iMac 27 qhd workplace on an 4K canvas (the way *retina* it's implemented on Linux and Wincrap) image quality dependents on this more than on pixel doubling *technique* (useful for legacy applications).
In a way or other I think the next iMac 27 to go upto 4K, a 5K display still likely but as an Mac store exclusive from 3rd party brand (as sharp) and only nMac Pro compatible thru dual display port MST.
In this regard we will see:
4K iMac 27 *retina* same form factor as current, with Haswell update and nVidia 970/980
4K cinema display, w/o Thundebolt (unless TB3 it's the Cuppertino' rabbit) maybe having an separate TB2 Port for peripheral support (usb3,lan, daisy chain tb2), compatible with 2014 iMac, 2014mini, rMBP15 and 2013 Mac Pro) .
5K display from 3rd party as Apple's exclusive and Mac Pro only compatible thru dual display port or single Hdmi 2.0 @ 30hz (HDMI 2.0 will be one of the updates for the L2014 nMac Pro, so if this monitor it's HDMI2 only gone hell Apple the nMP L2013 will become obsolete) .
The updated nMacPro will include from 6c Xeon and DDR4 plus ATI D310,510,520, HDMI 2.0, and maybe 10gb nic (following workstations trends).
The mini still a mistery if will be updated or replaced by another product(s) (name it Mac Nano and or Mac w/o monikers), this new product would include from Haswell i5/i7 and integrated gpu to Haswell-E with dual GPU (this it's only speculation).
Another *maybe* it's the iMac 21.5 if updated to retina adopting qHD or stay at hd until replaced by an 4K iMac 21 on 2016.