EDIT: Clarification (apparently, the above is cryptic enough to warrant a downmod): in any software project, the founders of the project can quit, go broke, die, or just be much less active. And no one can guarantee that it won't happen.
If the software is GPL, you have the guarantee that when that happens, some more developers can jump in and take the lead. If no one do so that means only one thing: no one cares enough (including users, which could hire someone). Good riddance. Example: CVS is dying because everyone is willing to switch to SVN.
If the software is proprietary, that possibility may be denied by the original founders. Which means a proprietary software can die while people still care. Example: old versions of Windows or Word, or games on old platforms.
Which leads to the obvious question: for any proprietary software, can you guarantee that it won't die before no one cares? I think not.