While it is a shame it was never brought back, at the time it was removed it was unavoidable since the bandwidth required for 5k was beyond what could be carried across a single display port cable.
Displayport 1.3 which supports 5k 60fps became widely available with the NVIDIA 900 series just 5 months after the 5K iMac released. AMD followed suit 1 year after.
They could have very soon added support for it, maybe even launched with DP 1.3 support if they worked something out with AMD.
