It's funny you mention it actually.
I'd be wary of any company that is growing so rapidly and that is dependent on one source for most of their revenue. A company will not grow at that rate forever, and if their current quality of service depends on their rate of growth, you could be shocked when you find them facing tough competition and start having to find other means to generate revenue.
It might take a significant event to hurt sales on Steam, but who knows what will happen?
Within 10 years, we could find 5 other similarly popular services as Steam, taking up some of the market share that steam currently holds nearly exclusively. It's certainly possible for Microsoft or some other guy to come in and offer something even more attractive than Steam. It's not as if Valve are the only clever company out there.