Sorry but I doubt they're even thinking about it that hard, at this point this post nails it [0], Valve just doesn't care about making any sort of experience that isn't maximum profit for minimum effort and isn't based around a in game item economy.
It certainly isn't out of worry that they'll disappoint their fans with a dated gametype. They look at Portal 2, a beloved game as a huge waste of their time. They're not a company that makes money from art, gameplay and storytelling anymore they make money from virtual items.
But don't be sad, plenty of companies still care about telling single player stories in videogames: Nintendo, Guerrilla, Rockstar, Kojima Prod, Naughty Dog, Sony Japan Studio etc
The concept isn't as dead as Valve believes it is.