I've got to second this. I mean the GBA came out in what? Summer of 2001 and the DS came out in late 2004. It was basically relevant for about 3 years.(And the DS was better at playing GBA games than the original GBA.)
2002 or so was when I first started going to video game forums, and I recall seeing NPD threads with GBA on top pretty consistently. It's weird looking back and seeing them introduce the DS while it was doing well, but at the time the DS was a gamble. They marketed it as a "third pillar" and it didn't do so hot in the first year, while GBA games continued to come out. It ended up paying off in the end, probably due to the DS Lite redesign and some strong titles in its second year and third years. At a certain point, it became silly for Nintendo to continue supporting the less powerful GBA while the DS was selling so well.
Who knows though, maybe if the DS hadn't come out the GBA would have been crushed by the PSP anyway.