HL3 kinda happened though, but it was called Half-Life Alyx. And while it wasn't a conventional FPS like HL1 and 2, there's absolutely no trace of GaaS in it.
Alyx is a bit of an edge case because they needed a VR showcase, and it's unlikely that a PC VR game (even a Valve one) could have sustained a healthy multiplayer population. Regardless of the reasons why it happened, it's the one and only singleplayer title they've released in the last 14 years, which neatly aligns with them discovering the joy of lootboxes 15 years ago.
What about Aperture Labs Desk Job as a demo for the deck? Full self contained single player story.
While it was more a tech demo than a full game, this one was a great game anyway.
To say Alyx was just an "VR demo" is pretty sad and reductive. Even today it's the one of the best VR games to be released in both fidelity and performance and as VR tech continues to improve its truly aged like fine wine
I called it a showcase, not a demo. I know it's a full-blown game in its own right.
Alyx is a great spinoff, a mindblowing tech demo, and a thrilling prequel. It is not Half-Life 3.