I think an important bottom-line to this is that using an application you do not own, on a system you do not own, is not like running a program, it's like having a service done for you. Despite the web application misnomer, Google Reader was a service more than it was an application.
So all the other rules that apply to services apply here as well. The commercial entity behind it can simply decide to end it -- just like a restaurant can decide to stop doing deliveries for whatever reason, including the manager simply not wanting to do it anymore.
This isn't a disaster to anyone. It can be quickly turned into a disaster if you start basing your whole business model on it. Like when you decide to start a food delivery business that receives orders for restaurants that don't do home deliveries, but you only take orders for one restaurant and as soon as that closes, you're out.