The real issue here though is it's a solution to a non-existent problem. This was potentially a problem before HDMI was a standard. That's no longer true though. Modern HDMI does video, audio and has an ethernet bus standard right on the cable.
All Apple is doing is proprietary lock-in here - it's not more flexible in anyway, where as HDMI is signal compatible with DVI and DisplayPort.
_Modern_ HDMI supports ARC, 3D and Ethernet. The early versions didn't, and you have to upgrade components to get the functionality. Apple is solving a real problem here: next time the HDMI consortium adds another random feature to their bus, Apple can just ship a new adapter that will work with all Lightning devices since none of the HDMI hardware is in the phone.
But you have to buy new HDMI cables every time the standard is upgraded. Right now the ethernet bus is only 100mbps, and it only supports 4k at 24fps; when these are improved = new cable. HDMI also can have latency issues, if you want to play something like RockSmith you need to use an analog output to circumvent it.
All Apple is doing is proprietary lock-in here - it's not more flexible in anyway, where as HDMI is signal compatible with DVI and DisplayPort.