This has turned into a bit of a Duke Nukem Forever unfortunately. If it did ever come out, would it now will feel like too long has gone by to appreciate where it left off, and if the project was restarted multiple times, would the end product now be a disappointment? I feel what happened with EP3 has been exactly like DNF. I think the project started and then lost its way for various different reasons. I even believe Valve got new teams to work on it and wasn't happy with the work. This is why Gabe doesn't share an ounce of enthusiasm anymore about it, in fact he asks people not to ask him anymore, whereas before he was quite happy to say it was in the works.
Sadly, I feel that Valve, and I hope they prove me wrong, are just a platform for games now through Steam, and all the work they do now is updates for csgo and tf2 which gives them a steady income. I think they appear quite happy & content continuing like that for the foreseeable future, as they have expressed little desire to really do more than that, and this has been going on a good few years now.