For a video game distributor they're worth a whole lot, around $2-5 billion dollars. But that's still a fraction to what similar services like Netflix, Hulu and other subscriptions are making.
They're big enough and have droves of dedicated fans to introduce such a system and live through the initial stages of grief many would have if they ever did reveal such plans.
The big thing they would hope on is complacency. Subscriptions, reward programs, promo deals are so common place nowadays if a Steam premium service failed it'd be a cold day in hell.