They also face a more competitive environment now than possibly ever before. There are other game stores/fronts like Epic's and Activision's, and the console giants are coming for a piece of their pie (Xbox Game pass and Sony's equivalent). There's also the ever-looming risk that Apple decides to go after their market with Apple TV/iOS somehow.
If I was an executive at Valve, I'd probably be happy with the current success, but nervous for the future.