They kind of did, with their sudden pivot from primarily making singleplayer games to almost exclusively making F2P GaaS titles the instant they got a taste of lootbox money. Half-Life 3 and Portal 3 will never happen because Valve makes 100x as much money with 1/100th of the effort by peddling Counter Strike skins.
HL3 kinda happened though, but it was called Half-Life Alyx. And while it wasn't a conventional FPS like HL1 and 2, there's absolutely no trace of GaaS in it.
Alyx is a bit of an edge case because they needed a VR showcase, and it's unlikely that a PC VR game (even a Valve one) could have sustained a healthy multiplayer population. Regardless of the reasons why it happened, it's the one and only singleplayer title they've released in the last 14 years, which neatly aligns with them discovering the joy of lootboxes 15 years ago.
To say Alyx was just an "VR demo" is pretty sad and reductive. Even today it's the one of the best VR games to be released in both fidelity and performance and as VR tech continues to improve its truly aged like fine wine
They tanked ultra-high-value items, which were primarily traded off-the-books because their value exceeded the official Steam marketplaces $1800 price cap. Bringing those prices down is good for Valve because it means more trading activity will happen on the official market, where Valve gets a cut of every transaction, rather than on third-party exchanges, where Valve gets nothing.
Simply raising the $1800 transaction limit and $2000 balance limit would have been far less disruptive, but that may have put Valve in financial regulators crosshairs. There's surely a reason why they chose those numbers in particular.
They kind of did, with their sudden pivot from primarily making singleplayer games to almost exclusively making F2P GaaS titles the instant they got a taste of lootbox money. Half-Life 3 and Portal 3 will never happen because Valve makes 100x as much money with 1/100th of the effort by peddling Counter Strike skins.