Who says the old DS can't exist for a few more years yet?
The PS2 is still being sold, and this is year 9.
They can continue to sell the current Wii and DS both for a few more years even if they release higher-end options for those who want that.
If Nintendo doesn't offer that option then Apple and Sony and Microsoft will and Nintendo will lose those who want something new. So they can make a choice to either offer that, or get out of the way and let those other companies take chunks of their audience basically without any resistance.
Not everyone just bought a DSi this year. A lot of people last bought a DS in 2005 or 2006 ... and in the next year or two are likely to want something new (not just a revision for grandpa). And if Apple or Sony or someone else is going to offer that whereas Nintendo isn't, then quite frankly they deserve to lose that business.