[quote name='62t']No. When Nintendo announced GB Mirco and DS the GBA SP was selling like gang buster. A lot of people questioned them why release a new version when sale was strong for the old system. In fact even after DS and Micro came out the GBA SP sold a ton of systems.[/QUOTE]
I think we can ignore the Micro; given that it's the only console I've seen on sale at Big Lots, we can assume it wasn't a rousing success.
As for the rest of your point, keep in mind that at first, the DS was never intended to replace the Game Boy line - Nintendo was referring to it as a "third pillar" besides their consoles and the Game Boys, which makes your argument moot.
With their handhelds, Nintendo has never been shy about updating the hardware; that's why we saw the Game Boy Pocket, the Play It Loud series, the SP, and the DS Lite. They usually don't release entirely new systems until the old ones are nearing the end of their lifespan, though, and the DS hasn't really shown any signs of slowing down.
The Lite has been out for a year and a half. Isn't it a bit soon to replace it?