In a free market transactions needn't be limited to simple currency for product exchanges. They can come with whatever strings, of whatever enforcability, the parties place on them.

So here a buyer had an expectation of future development and upgrade, some of them perhaps to be paid for. They invested time and attention learning a new interface and functionality, thinking those upgrades would provide returns on that time invested. Investments like that are how communities are built.

I know squadoosh about the small app market or Sparrow's marketing. But if there are a lot of buyers with expectations like that, then I think you have to speak to those when you are selling or yeah, there is an implicit agreement there. If Sparrow disclosed future plans or roadmaps then the agreement moves toward explicit.

Such obligations aren't enforceable, and what's customary within a market is obviously fuzzy. But that doesn't mean those obligations don't exist or aren't ethical.

Again, I know nothing about this case -- but if there was some understanding that the company would this, that or the other, then it is worth wondering about the ethics of a large company doing an acquisition, to hire people, in the knowledge that the deal breaks some tacit deal with a community.

I agree that some kind of support should continue.

But in the case of an email app, focused on simplicity and ui design; not feature sets for power users, i dont think this expectation can reasonably consist of new features.

On the other hand: the value was in the ui design, not some dificult technological challenge. If there is actually a viable market here, chances are more than one player will follow in their footsteps. UI designs are easy to copy.

And until then, the kind of support one can reasonable expect, will continue: bug fixes.