I do think there's a distinction between venting in private about your disappointment that a story isn't being finished, and making demands on the author.

As much as yes, I do think that starting to publish a WIP creates some kind of expectation that the story will be finished, I also think that it is entitled to make demands of a specific author - to post the next part now, now, now, or to finish a story regardless of changed circumstances.

Feeling disappointed (maybe even a little let down) is surely a perfectly reasonable response to knowing that you will not be able to spend any more time in the world of the story? I may complain that my favourite show is cancelled maybe even on a cliff-hanger, is that automatically entitled? How you respond to that disappointment is where the entitlement lies.

I also tend to disagree about kinkmeme fills, I think they are one of the forms of writing where someone does have the right to feel cheated at an unfinished story - not every random reader, but the person whose prompt it is. Not as much as if it's a gift exchange, but the story is being written for that person, and a half-written story is very liable to discourage others from filling the prompt.

The suggestion that several people have made about releasing the story for someone else to finish might hopefully fix that problem - there's no guarantee, but a reasonable chance that the prompter will get their story finished.

In the end, what I think matters is that we treat each other with respect, both readers respecting authors and authors respecting their audiences.