I think people are being naive if they expect some mythical Half Life 3 to pick up exactly where HL2 left off after a 15 year gap with a completely silly amount of over-hype and without disappointing anyone. As we've seen many times before where there's been +15 years between sequels, you just can't go back for a lot of games without the new game ending up feeling out of place / like it belongs to a different franchise. What really "makes" games isn't the Company logo stamped on it, but rather key individual people, eg, to me Torchlight felt more like a Diablo 2 sequel than Diablo 3 did because it was developed by Max and Erich Schaefer (creators of Diablo 2). That it was made by Runic and not Blizzard didn't matter that much.

Likewise, most of Half Life 2's original dev team have moved on. Even wondered why the Tallboys in Dishonored look a lot like the Striders in Half Life 2? It's because Valve's lead art director (Viktor Antonov) who created the whole City 17 concept has long moved on and now works on other games like Dishonored 1-2 made by other studios. Half Life's main writer (Marc Laidlaw) has left. He even posted the script to HL3 precisely because he knew it isn't going to be made. The lead designer of Portal (Kim Swift) has also moved on and made other games like Quantum Conundrum and Star Wars Battlefront II. None of them work at Valve anymore and Valve are now more interested in Dota 2 / Artifact like games due to the monetization. In short - Valve simply aren't the same company they once were, and even if they wanted to do a HL3 / Portal 3, there's hardly anyone left from the original HL2 / Portal team to give it the same feel.