They were running for a long time (months? over a year?) on a single rack in a single datacenter. Eventually they scaled out but the word is eventually. I think that summarizes both sides of this debate in a nutshell. You can move off of AWS but unless you invest a lot you will take on increased risk. Maybe you'll get lucky and your one rack won't burn down. Maybe you won't. They did get lucky.
> Maybe you'll get lucky and your one rack won't burn down
Given the rates of fires in DCs, you'd rather need to be quite unlucky for it to happen to you.
Hm.. I wonder what the risk of a rack going offline is? Maybe 5% in a given year? Less? More?
Compared to all the other things that can and will go wrong, this risk seems pretty small, but I have no data to back that up.
From the story, they seem to have kept the option to fallback on AWS.