Well, this is what we get for buying closed source apps. Let this be a lesson - buy just apps that are open source with a license that allows redistributing the code.

You cannot expect a developer to support his app forever, just from the small amount of money he initially received. Well, he should do that, but if he doesn't, you can't do anything about it.

If the app was open-source and worth it, someone would definitely continue the work (or you might) even if the dev abandoned it.