Earlier quoted context omitted.
I don't use Sparrow so please correct me if I am wrong. The applications are still fully functional and are working perfectly fine until Apple changes anything they heavily depend on or something replaces imap. You own your copy of Sparrow and it still works. Imagine you buy a BMW and in 10 years you cannot get the required fuel anymore. Does this mean you only pay for a new car when you get all plans for the engine…
For something as critical as email, "fine until Apple changes anything" is not good enough, given that Apple has a habit of changing things.
Apple doesn't have a habit of changing things. If anything, I feel that Apple is too conservative when it comes to making changes.