I think what has held HL3 back is Valve thinking they need to deliver something _more_ than just a continuation of HL2. HL2 was meant to showcase physics and the source engine, Valve needed something like that for HL3. Alyx I think was supposed to be that "thing": VR, the next frontier.
Unfortunately, I think most folks would have been perfectly fine with HL3 being a continuation of HL2, myself included, so now we have to wait for Valves hubris to subside.