Well, sure, I paid $20 or whatever for Sparrow (twice, actually), and I get the impression that tons of other people did too, but it didn't make much difference

Well, sure, I paid $20 or whatever for Sparrow (twice, actually), and I get the impression that tons of other people did too, but it didn't make much difference, did it? And it never will -- there are always going to be developers who are in it for the money and will leave you high-and-dry (which is heartbreaking, in a very-small-violen way, but fine). My own takeaway is if you don't want your apps to disappear, use open source apps. (Except for some reason, open-source apps seem to tend to be terrible, UI wise. Not sure why this is, though I have a couple half-baked theories.)