Standard overlap between a highly popular system and its successor. Basically if you've got a tremendously successful system like the GBA, you don't deliberately kill it off in favor of the next thing, you let it run its course. As was the case with GBA / DS. The 'third pillar' strategy implied supporting the two systems in parallel for the long term, which Nintendo never intended to do. DS was always going to be a replacement for GBA. --- floating upon the surface for the birds.