I can't think of any example. Maybe some other sort of software than the ones for programmers, say, graphic editors - there are some that are pretty hackable but they aren't being hacked too much despite having users.

That's why it's so striking, that the developers here seem to declare to fight this "almost a natural law". They want something hackable, yet they want the thing to have very specific purpose, but I think, if it will succeed, it will take life on its own. I think they should plan for it and embrace it, instead.