Valve isn't seriously in the business of making video games anymore, and they haven't been for some time. With the exception of The Lab—a set of free mini-games that shipped in 2016 with the HTC Vive—Valve hasn't actually created a title since Dota 2, back in 2013. It's worth noting that many of Valve's franchises, including Left 4 Dead and Portal, started out in the hands of other companies—companies which Valve then acquired. Similarly, Dota 2, Day of Defeat, Team Fortress and Counterstrike all started as mods—mods which Valve then acquired.
Do you notice a pattern here? Valve's only game franchise that they conceptualized and developed from the very beginning was the Half-Life saga, which launched the company into the big leagues. Since then, they've moved on to more profitable ventures. It obviously isn't out of the question that they'll continue to release sequels for some of their games, and they obviously continue to have developers, writers, and designers on their staff for a reason, but it seems likely that's for DLC, updates, in-game items, the Source 2 engine, the Vive, etc.