Seems like the solution to this is to just allow for the iMac to work as a monitor.
As I understand it, the reason it does not is that actually driving a 5K monitor at 60Hz requires specialized hardware. With the iMac, Apple was able to design their own and tie it into the on board AMD GPU.
A standalone 5K monitor at 60Hz requires two Thunderbolt 2 ports so you'd take up both ports on the iMac as input and both ports on, say, a MacBook Pro as the output, which means the only TB device you could have would be the iMac itself. The Mac Pro has enough ports to dedicate two to driving a 5K monitor, which is why some likely use the Dell 5K monitor with it.
I believe even TB3 needs dual channels at the moment to handle 5K, though there are updates to the Displayport or HDMI spec (not sure which one) that will allow 5K over a single TB3 (and USB-C?) port, but I do not believe Intel has released a chipset with that support yet (might be slated for Kaby Lake?).