Wow. I'm not the penultimate expert, but from everything I have read, with TB3/USB-C, and now with Skylake processors supporting Displayport 1.3, there should be absolutely no issues in terms of actual hardware limitations that should prevent this from working. In that case, it's merely Apple's decision... and it is an extremely bad one. They just lost an iMac sale to me because of it. Target display mode is EXTREMELY convenient - I still use my 2010 27" iMac (am using it as a monitor with my MB pro as I type) - and would have happily upgraded it to a 5k monitor/iMac that I can use as standalone when I want to leave my MBP at the office - but as it is, what is the point of upgrading? I guess for people that only plan on using it as a standalone desktop, ok - but it's hard not to feel this is just a customer gouge on Apple's part now. At this point, it seems to only be a matter of software - not hardware. Disappointing.