The DS and GBA won't coexist side by side. The DS has to take over for the GBA. Seriously, I don't know how Nintendo could possibly get away with that when Sega was eaten alive for doing the same thing. They released some fairly diverse products with the 32X and then the Saturn, but it was all viewed as generating from the same line, so it bit them in the ass. I don't see how anyone could see the DS and not see yet another Gameboy.
I don't think the DS will eat into GBA sales, but then again, I don't think the DS has much time on the clock, period. I see it falling by the wayside, and the GBA sales continuing on their current pace for at least the next year. Can the PSP eat into the GBA's monthly marketshare by late 2005? That's the question. It can't compete directly in sales, just no way. But if by next holiday season, the PSP can match the GBA in sales, that will signify the turning of the tide IMO. Beating the DS outright isn't really an option, it's what has to happen, otherwise the system is a failure. PEACE.