Displayport 1.3, the answer?

You undersell, DP 1.2 can do 4k @60hz. DP 1.3 can do 4k at 120hz. Would require a multiple Displayport adapter for those who have not purchased there Graphics card in the future. Unless some parts of 1.3 like VESA stream compression can be had with a mere driver update. VESA stream compression is supposed to double effective bandwidth again so you could have 5k 120hz. Have not found out the latency on that though as its only half compression that implies its made with latency in mind.

HDMI 2 only just court up with DP1.2 bandwidth which has been around for years.