I thought the numbers showed that even DP1.2 and Thunderbolt 2 didn’t have the bandwidth to support resolutions much higher than 4K ones (such as UltraHD, 3840 x 2160 @ 60Hz), so that ~5K (doubled 2560 x 1440 displays a la the current 27” ones) would have to wait for DP1.3 (the standard is not even complete, the earliest this could happen would be sometime in 2015) and a future Thunderbolt interface. And even Intel’s announced plans for the Alpine Ridge Thunderbolt controller that’d support 40Gbit/s speed has no announced timeline and (according to Wikipedia, anyway) does not support DP1.3, just DP1.2 (HDMI 2.0 is similarly limited to sub-5K resolutions for 60Hz display modes). Perhaps that’s merely because the DP1.3 spec isn’t finished yet, and Alpine Ridge will support it once it’s finalized.
In any case, I think if we see desktop Retina displays, they’ll be limited to 4K (whether UltraHD or wider-screen cinema 4K standards) and will use retina scaling modes to approximate a doubled 2560 x 1440 (or higher) resolution, a la how the current 15” Retina displays in the MBP work. The technology just isn’t there for larger panels yet.