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.
Sometimes when I think about how anxiety-provoking it must be to even _think_ about making Half-Life 3, I wonder if Valve might not be better off building something that completely defies expectations about what "Half-Life 3" should be instead of trying to meet those existing expectations and then inevitably disappointing a huge number of people even if they produce something pretty damn good.
What could it look like instead? Anything! I think a Divinity-style RPG Half-Life would be heaps of fun. Or a hero-based RTS that still has Gordon Freeman centre stage. I think basically any genre that can tell a story would be fine.
Of course this guarantees disappointing a lot of people, but I feel there's a lot better chance of Valve producing something creatively worthwhile this way than trying to make a "vastly better Half-Life 2", which is what most people seem to be waiting for.
This pretty much exactly describes Alyx. They defied expectations about what Half-Life 3 should be and disappointed a large number of people (who were mad that it was VR exclusive) despite producing something incredible and unprecedented.
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.