What kind of message would that be sending to future service providers? "Don't give people access to your data for free, because if they like it too much they'll come back with guns and demand it forever."

What if Apple eventually discontinues touch devices and iOS. Should iOS developers have a right to demand iPhones be made a national treasure so that game developers can still use the platform?

Sorry you invested so much into Flickr's API, but you had every chance to decide for yourself whether you should build on their platform.

How does Flickr free vs. Pro relate to API continuity guarantees? Flickr could turn off all API access and you would lose none of the advertised benefits of Flickr Pro that you pay for. If you want guarantees around the API, my initial suggestion of offering to pay Flickr for such an SLA still applies.