The following was meant as a comment of Valve's page here on ModDB but its size grew in such way that I decided to post it as a mini-article.
It's sad to see how Valve transformed from a company that makes games to a company that makes money. Is this the 'thank you' we are getting as players for buying, playing and otherwise using their products? Apparently it is, since Valve no longer NEEDS Half-Life!
When Valve was a struggling developer, they needed a hit title and that title would become Half-Life. A lot of us were in their late teens when this game hitted the shelves and as always, money was a bit of an issue with low-paid jobs and such. Never the less, we still bought the game plus the expansions and secured Valve's future. Then Valve came with the idea of Steam but for that, they needed another hit title, so a sequel for Half-Life was developed and that became Half-Life 2. Without Half-Life 2, Steam would have never been so popular as it is today. For example, take a look at Direct2Drive and similar from that era (not counting today's Origin, GOG Galaxy, Uplay). All were less popular because of the lack of a hit title that would have sold the platform. Half-Life 2 further secured Valve's financial position.
After Half-Life 2 and its source ports (Counter-Strike: Source, Day of Defeat: Source), Valve released Episode 1 and 2 with the sole intention to keep the interest in Steam peaked. After all, what good will only one 'hit' title do for the platform? Nothing off course, so the only reason for the episodes was to generate enough 'buzz' for the Steam platform as a selling point. After the release of Episode 2 in 2007, things began to silence real fast. Occasionally there would be some comments here and there about possible things incorporated within Episode 3 but after a while, even Valve realized that they could not keep telling the same lies over and over again.
In the meantime, the only thing Valve "did", was buying other developers content and put a big Valve sticker on it. Games such as Left4Dead (plus sequel), Portal (plus sequel) and CS:GO were all developed by other developers, who were then 'acquired' by Valve Software and subsequently there was a "new" Valve product. While others were busy with new games, Valve abandoned the Half-Life series in order to pursue other money generation endeavours. In the early 2010's, the phenomenon of microtransactions grew to great heights and Valve decided to step in this bandwagon by introducing it into some of their products. Team Fortress 2, CS:GO and DOTA 2 all contained these 'microtransactions' and has become one of the large contributors to Valve's revenue, besides the Steam sales and the thirty percent cut they take from non-Valve developed games.
This is why I believe there will never be a Half-Life title since the interest isn't there anymore, since Valve financial future has already been cemented in concrete. Furthermore, others are now doing things with the Half-Life universe with the Crowbar Collective finishing up Black Mesa with the upcoming Xen chapter and the (still in development) mods "Guard Duty" and "Operation: Black Mesa" (and others), retelling and bringing the well-known story into the 21st century with all the capabilities nowadays possible (and doing an excellent job, if I may add).
Another reason why I believe that there will never a Half-Life 3, is its story and the way it was abandoned. Half-Life was set in the Black Mesa Research Facility and Half-Life 2 and its episodes, were set in City 17 and its surrounding environments. Half-Life 3 should have introduced us to a whole new environment (let's take the fight to the combine!) instead of now requiring to mop up elements of its predecessor. Episode 3 should have handled that and concluded Half-Life 2's City 17 storyline and no recently returned writer (Erik Wolpaw) will be able to change that, despite all the HL3 memes and conspiracies!
I dare to say that Half-Life is dead and after a decade, I do say it's time for other media such as gaming sites to pick up this mantle as well and declare the series dead with a nice editorial piece talking about the highs and lows of the series. It would become the end of a beautiful era, the era that is called Half-Life . . .