This was initially kinda justifiable; when the first 5K iMac came out there was no commonly available connection that could do 5K (the iMac used a somewhat overclocked DP link internally, I think), so their only option would have been two DP cables (a configuration that then also had very limited support).

That excuse really went away with TB3 and newer iterations of DP, tho, and it's pretty disappointing that they didn't bring it back at that point.

I've seen projects online about converting 5k iMacs into external displays by essentially ripping out the insides and putting in a driver board instead - I wonder what the relative cost (/value proposition) of that is compared to these new displays if you don't need a webcam?

Ouch! I always thought that was an awesome feature and didn’t know they got rid of it.