In my view, Half Life 3 did come out. It was just called Wolfenstein. For all practical purposes, there is no longer a HL3 shaped hole.

Given all the prep work HL:Alyx did, I wouldn't be surprised if there eventually was a HL3, it's just going to take another decade or so until enough people have a VR headset(or future disruptive equivalent) to make it worth it.