Considering that Gabe Newell, during a podcast with Geoff Keighley last year, specifically told him in response to questions about the future of the series that because of what Valve has learned over the years (and in particular since shipping Portal 2) that they weren't interested in doing what he called a "super classic kind of product" unless there was a significant enough demand for it within the company itself, it seems like the writing was on the wall.
Unfortunately, Newell made it pretty clear where Valve is at right now. That the company is focused on the community-based experiences that have helped make the company so successful, rather than the traditional single-player experiences that made the Half-Life series what it was. He even comes out and calls doing another Half-Life a "bad choice" that it "ignores everything we've learned." Anyone still holding a torch for the series only has themselves to blame.
Of course, that doesn't mean the franchise is buried. However, based on what Newell has said on the matter, there's little doubt that even if it did come back it wouldn't be as we knew it but reflect the current direction of the company. Counter-Strike, Team Fortress, Left 4 Dead, Dota. It would be something that reflected the community driven, market based, competitive standard Valve has built around its brand. Valve and Newell both are clearly more interested in services and hardware than the traditional experiences that once typified their offerings.
Best not to expect anything anymore.