I was really hoping this would come back once it was feasible (at the time it came out, the only vaguely well-supported way to do it would have been two DP cables, but later versions of DP and Thunderbolt 3 and better can handle 5k without trouble), but, given that the 5K iMac is now gone, I wouldn't hold out too much hope...
The Apple Studio Display certainly improves on it, but at ~$1600 it costs more than a midrange 24" iMac.
It's too bad that the iMac 5K didn't support Target Display Mode. Maybe Apple will bring it back someday along with a new 27" iMac.