It seems like this was a technical limitation more than a business one. Apple had to do some very non-standard stuff for 5K and that's when they stopped support

It seems like this was a technical limitation more than a business one. Apple had to do some very non-standard stuff for 5K and that's when they stopped supporting the target display mode. See the article as well - the only was the author could get it work was with two display port connections, so if Apple were to support target display, they'd likely have to put in two display ports in the back just for that.