I'm not sure I really agree with the reasoning on why HL:Ep3 was never developed. Sure, Valve game usually revolves around some game-changing mechanics, but at the same time, Episode 3 was more than just a game, but a sendoff of a big franchise, at least for a time, and a promise to their fans: that 3 episodes are made.
I think that this was one of the signs of the cracks forming in the game developing sector of Valve. It's been well known that the dev team within Valve has slowly developed to a more toxic environment where veterans vetoed a lof of stuff from newer employees, leading to stagnation and developers getting silently shunned for working on projects like TF2 because it wasn't considered "profitable", when there still was, and is, a sizeable community revolving it.
I would have said playing it safe and making a sort of "best of" of popular mechanics used in the previous games would've been the best choice, because that would be the best possible sendoff for the series of episodes. After all, at that point people mainly wanted a conclusion to the story arc, a moment of "this was the HL2 era, thank you for playing", rather than something completely new.