Your definition of "downfall" is your downfall.
Its the classic reason why "win" & "lose" cannot be easily applied to the games industry - it just doesn't make sense.
Just because the 360/PS3 are NOT outselling the Wii, does NOT imply downfall.
I think the current sales levels make perfect (common) sense - the Wii is simply a console designed to appeal to non-gamers, hence it has a much larger potential sales market. Note that it happens to ALSO appeal to (some) gamers - but big deal.
(Do I enjoy seeing the really good games NOT come to the Wii? Hell no!)
End of story.
Gesta Non Verba
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