For any substitutable software, open-source will eventually dominate. (Substitutable: Can be substituted for a different software package that does the same thing. Examples: AIM and Trillian, Word and WordPerfect, Photoshop and The Gimp.) (Eventually: "once the open-source package is good enough", which will happen *eventually*, it just might take, oh, another decade or so.)
For any non-substitutable software . . . that's a different matter.
If Halo 3 (for example) isn't open-source, there isn't really a game you can substitute for it. I mean, sure, someone could make "GNU/AlmostHalo", but it *won't be Halo* - it can't have the same plot (copyright infringement) and it won't be made by the same people. There's no way to incrementally improve a "proto-game" until it's just as good as any other game - it just can't be done.
And for MMORPGs, there's the entire issue of "who pays for the central server/tech support".
. . . Yay! Open-source won't make my job obsolete!