its not viable. that's the point.
thinking about this:
you have to rent a Audio R8 for 365 days, But you can only use the power 4 days a year when you go to the track, would you rent said car? No you wouldn't unless your mad.
should they pay out millions for just a few days of issues on their store? No. its Silly Wasted Money. ID much rather tem spend the Extra Money they save on Steam UI/Feature development
Ev0 covered this above, but fewer and fewer companies own their own servers on-premises now. Cloud hosting such as Amazon's AWS and Microsoft's Azure can be cheaper, more secure, and have higher up-time than local servers.
But crucially, they can also be configured to automatically scale based on traffic and you only pay for what is being used. So when you have a light amount of traffic hitting the servers then it scales down to a small number of boxes to save money. When large amounts of traffic turn up (such as the start of the Steam Sale) then they automatically scale up to deal with the load. If you're interested in reading up on it, Amazon have loads of info on it, starting here http://docs.aws.amazon.com/autoscaling/latest/userguide/auto-scaling-benefits.html