It seems to me that the Wii and DS are being replaced more due to demand by developers than by players. I don't think that same demand is there for PS3/360 successors, which pretty much leaves their main reason for updating as "steal Nintendo's thunder".
I think it's more in line with the typical Nintendo strategy. Whereas Sony & Microsoft have other divisions that make money for them, Nintendo just focuses on gaming alone, so when one of their systems starts to drop in sales, it's time for the next. Personally, I think Nintendo made the right call with the Wii in the first place. Now HD is much more mainstream, accessible, and possibly cheaper to develop, they can release a more powerful console for less of the cost and hopefully bring all the people they gained with the Wii over to the Nintenterprise.
Microsoft rushing out a system is pretty much what got them in trouble with the 360 and the [acid burst] poor god awful hardware it first started out with, so hopefully they are a lot more careful this time around with their next system. I think they want to do it cause Nintendo has the Nintenterprise and Sony has the NGP.
As for Sony.....I honestly don't know what's going to become of them. Due to the PSN debacle, they could have some seeeeeeerious lawsuits coming their way, and they need the NGP to actual make a profit, and quite frankly I don't see how it can. I don't see how that system is going to be under $350 at the launch date and besides the hardcore gamers & tech nerds, I don't see anyone buying it.
But we'll see at E3.