I think a lot of people would have refused to pay out of principle; it's basically a hostage negotiation. "Give us $10, and you can keep Google Reader."
That's why you need to get the users to ask you to charge for it. Say, "Sorry, we have to shut down Google Reader because it's drawing needed resources from our other projects," or whatever. Then when people say, "NO! Please, just charge for it! I'd be happy to pay $20 per year for Reader!" you benevolently listen to the community and agree to transition to a paid service instead of shutting it down.
