For a game widely considered to be one of the best I find it surprising Valve hasn't just yeeted dlc at us over the years of new content. I don't need a story, I just need coop fun. Weird since there's been SO MANY fan made mods.
They barely take care of TF2, their golden child nowadays. Its got like one dev working on it, and most of his work is implementing community made stuff. And TF2 is still making them money.
On top of being among the top 10 most played games on Steam to this day, just last year it reached its highest player count of all time. It also has a very active community to this day.
Although it may be more appropriate to call CS its golden child, TF2 is probably its second best
Plenty of the mods (portal stories Mel, portal reloaded) have plenty of playtesting. Valve already did most of the work with the base game, making sure the mechanics all function properly. But I think it's just as feasible to design a good puzzle with 5 playtesters as it is with 100.
Because someone did the math and decided that the cost of development plus the cost of ongoing support was less than what they'd make selling the DLC, or was less than what they'd make selling TF2 hats or whatever