"the bitter cliffhanger from hell."
"the most anticipated sequel of all time."
"no matter how good the game ends up, it'll be dooming itself to become the next Duke Nukem Forever, completely crushed under the weight of its own expectations."
"one of the most highly-regarded series of all time"
There are your problems. Can Valve make good games? Yes. Can Valve make great games? Yes. But when they are faced with these expectations and problems, Valve would be idiots to try and make this game, because the game that people are expecting is impossible. If Valve makes one mistake, one hiccup, if the game isn't the best of the best in all fields, then people will shoot it down as a disappointment. It has gotten to the point where Valve has an impossible decision to make: Damned if they do, damned if they don't. Valve are making a game, that is a given, and whilst they probably have someone looking at the possibility of Half-Life 3, my bet is they are concentrating on a sequel for another game, or a new IP entirely. In the end I would rather be the company that never made the most anticipated sequel of all time than be the company who made the most disappointing sequel of all time.
What you might also forget is that Valve isn't a normal developer anymore. Sure, they too generate revenue from game sales, but Valve get a cut from every game sold on Steam, and that is probably enough money to guarantee that Valve never has to make another game again. However, Valve also generates revenue via merchandise sales, their cut from sales via the Steam marketplace, software licensing (Steam OS, Source), and soon to be hardware (Steam controller). Most other AAA developers only ever have one additional source of revenue, but Valve is in the kind of position that they don't even have to make another game to stay afloat, because they are constantly and consistently getting money from pretty much everywhere else. In other words, they can take as long as they damn well please on whatever game they are planning on releasing, because there is no pressure whatsoever.