I think install base affects mostly games with wide appeal, games like MKWii, NSMBWii, CoD, Pro Evolution, Guitar Hero, Madden, etc. Thus, most of these games increase their sales as the userbase expands.
I'm pretty sure if Konami released MGS5 tommorrow, it would sell exactly the same as MGS4, there would be no penetration. That's why we see games like SSBM, FFXIII, etc reach dizzy attach rates.
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