IMHO this understates the difference, severely. The open source app can be maintained by the community for as long as it has a community. That includes not just dealing with dependency obsolescence but also new features, redesign, porting to new platforms… anything that would have been handled by the original team.
The proprietary app, on the other hand, will be near-impossible to patch for even the most trivial of changes without the original source code or build environment—and let's not forget that distributing the modified version would be illegal; at best you might get instructions for patching your own copy. And of course there won't be any growth in the user community since there is no (legal) source for new copies. Existing users may be able to struggle along on their own for a while but this is ultimately a dead end without support from the copyright holder.