OS updates will break a small piece of functionality. Critical security vulnerabilities will be discovered that need to be patched. If the developer abandons the application because it isn't earning new revenue, the application will eventually degrade over time.

I don't have a good answer here, because I also don't like the SASS billing model for desktop software, but I recognize that a "one-time purchase" rarely actually terminates the relationship at the time of purchase. People expect ongoing bug-fixes, and security patches even for desktop software.