I think something to consider here is that Nintendo planned on having a 5 year life span which has been typical every generation up until now. Wii will easily be selling for another 3-4 years at some level but likely a new system will be out holidays 11 or 12. Nintendo never came out and said that but it's obvious. When Wii launched HD wasn't in a majority of homes but even in '05 they would have assumed HD would be the majority by '10. It didn't concern them (much) because they figured they'd need a new system by then anyway.
Considering Nintendo profitted from day 1 with Wii, there's absolutely no shame or loss in bring out a new system next year.
MS and especially Sony can't say the same. They need a 10 year cycle and so do most 3rd parties.
Nintendo isn't in any kind of problem here. Wii is still selling well and they are profitting nicely (although currency exchange isn't making it look good currently).
Conversely a new Console from Wii which the same impact of Wii or 3DS in 2011 could devastate Sony and hurt MS margins. Frankly, Sony and MS would rather see Wii having flat sales so it stays in the market for 10 years than continually dropping.
