In an actively maintained project, that bug could be fixed in a day (or at least within the week). In an actively maintained proprietary game, it could go out in a patch pretty quickly. In an abandoned proprietary game, the bug will never be patched. In an abandoned open source game, there's at least a chance you might be able to fix the bug yourself.

And that's one part of the freedom. Open source software will live as long as people are interested in it, while proprietary software dies very definitively.