I don't believe a sales drop to the 360 would affect the Wii - they are very different machines.
The main threat to MS come from Sony/PS3 - and they will make a huge move in the next couple of months. In fact, I don't think price is *that* relevant when comparing the 360/PS3 - its games/performance that is.
MS have to make sure they don't lose customers to the PS3. Their first target should be to take the PS3 out of the equation for this generation. If they don't - and the PS3 catches up (or even looks like it will catch up), this could be disasterous for the 360 (long-term).
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Once MS have shown they have turned this generation into a two-horse race, they can address the Wii. And the only way to do this, may be through a "next-generation" device that is more accessible/casual (similar to the Wii), cheap and powerful.
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I personally don't believe MS can actually come 2nd (Japan prevents this from happening) - but if they work hard, they can turn it into a really tight fight for 2nd place.