This feels a bit more like the power & water was cut off to the beach house, but I like taking metaphors one step to far.
It doesn't matter that its an app for Twitter, you've paid for someone to provide something they can no longer deliver.
One is "I paid for a service" and the other is "I paid for something to help me use a service".
So, no, if Netflix disappeared in this case, I would not ask for a partial refund. The TV still works as a TV, and that's what I really paid for in the first place. (If Netflix didn't disappear, but the TV app stopped working, I might be -- justifiably -- annoyed, though!)
This Twitterific thing is in no way comparable: the app only interacts with Twitter. That's its entire reason for being. If it can't do that, then it is worthless, and anyone who paid for it is perfectly in their rights to demand a refund. Hell, the Twitterific folks should be pro-actively refunding the unused portions of subscriptions; that's the only ethical thing to do here.
Now, if you had dial up and paid a subscription for access to a free phone number and they cut you off before your subscription ended, then yes you absolutely would expect a refund.
And, regardless, if my ISP stopped providing dial-up service, and I really still wanted dial-up service, I would have simply found another ISP to connect to. (After expecting my previous ISP to issue me a pro-rated refund for whatever subscription I was paying them.)
Also regardless, your analogy is bad, as the modem was a one-time cost. The issue at hand is that of recurring subscription fees Twitterific charges. If we were just talking about a one-time cost to buy the Twitterific app, I wouldn't be suggesting the refund that cost (except to customers who just bought the app right before the shutdown, perhaps; seems shitty to keep customer money when the product irreparably breaks a few days after purchase).
You bought a display that exclusively integrates into Netflix, it can only display Netflix content, but you have to pay an annual fee to the display manufacturer for it to continue working.
It's January and you've just paid your annual subscription. But on the 15th of Jan Netflix decides to stop allowing your display manufacturer access, and your display becomes a useless brick.
Having only just paid for your annual subscription you'd want a refund - you're no longer getting access to the service you were paying for.
We can keep coming up with anologys but the fact remains that the app developer guaranteed access to a service which they can no longer provide. It doesn't matter if its an app, a display, or anything else. Your agreement is with the app developer, not Twitter. Your payments go to the app developer, not Twitter. If the app developers entire business is built around hoping Twitter never closes off their access then thats totally on them when things go south.
Yes, they did close down third-party access to their platform, but that is completely not the same as those other things you mention.
I doubt most Twitterific customers have any idea what a third-party API platform is, let alone Twitter's shady history with how they've treated API developers. Your average Twitterific purchaser probably just thinks it's a premium Twitter app that lets them do Twitter things, and would expect that app to continue working as long as Twitter exists. (Which it still does!)
> On any reasonable viewpoint that's a risk accepted by the purchaser, not the app seller.
If your definition for "reasonable" is actually the definition of "unreasonable", sure. Twitterific's customers should not have to take on the consequences of the risk that Twitter would shut down third-party access to their API. That is a risk that Twitterific took on themselves as a risk to their business, and they should have a plan for what to do in this case. And that plan should not be "beg our customers to pay us for not providing a service anymore". It should include them pro-actively refunding the unused portions of any subscriptions (super gross that they didn't do this, double-super gross that they're begging for "donations"), and then responsibly winding the company down (assuming they don't have any other still-viable products that could keep them going). It sucks that they'd have to do this, but that's what can happen when you bet your business on someone else's platform.