about the GBA/SP

i got a problem, not a big one but it annoys me that the GBA has only been around for about 3 years now. So why the heck does Nintendo wanna make a whole new handheld that is seperate to the GBA. I dont get it. This is confusing me cause i cant see how Nintendo can continually make GBA games while still making DS games. The developers woudn't like to be making games for over 4 seperate handhelds. Sure its there job, but wouldnt it be a bit silly to be developing games like that? Some games may result in being crap because of a rush job, you know, the release date for it gets closer, they may have to drop the release back a few weeks... and it annoys us.