Nintendo understands the market of short-term gain. The Gameboy Advance line and system is slowing dying, but there is an untouched techno-savvy market that hasn't bought it yet. And there are devoted Gameboy loyalists that desire to have an easier Gameboy to carry around. Hence they created Micro. But, Nintendo knows Micro is a limited Product.
The same is true about the DS. Yes, the DS has been very successful, but it will not be the next Gameboy system...unless the PSP totally fails. So they Nintendo is already moving and reacting to the market as a short-term market. The new DS Lite will be released, and will be sold along side the original DS until the final batches of the original are sold. Then Nintendo will only create DS Lite for awhile, and eventually move on to the next Gameboy or release another iteration of the system.
This isn't to say the time frame for this is anytime soon. I think the DS easily has another year or two left in it. I see this system surviving until the end of 07. But you know that the GBA won't survive that long, so 2007 Nintendo better have the next Gameboy system ready for sale.