While I don't doubt that stores will be looking at phasing out GC games soon, I really do think that the Wii's broad adoption is driving a surge in the used games market for that platform.
I picked up MGS: The Twin Snakes a couple weeks ago and saw that there were three used copies in the store. The clerk, a GC owner, complained that this title always got bought out before he noticed it and bought it himself. I mentioned that there were two others, and they were both gone a week later. That's not the only GC title I've noticed disappearing off shelves.

"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.







