Sure. Most of DS supersellers could be done for GBA. Success of DS is based not on the dual screen but on all these quasi-games (nintendogz, brain trainer etc.) and simply stuff like mario-something.
Edit: I've just checked the hall of fame of DS and I think all supersellers could be done for GBA...
You mean Nintendogs, Brain Training, Mario 64 DS, New Super Mario Bros., Metroid Prime Hunters, Resident Evil, and coming games like FF12, FFCC, Zelda: Phantom Hourglass?
No. You can´t make 3D games on GBA and there is no touchscreen too. Why can´t we stay logical in this discussions?
Edit: I forgot to answer the topic question. Yes, Nintendo would´ve won. Look at the US were the GBA and the PSP still sell at the same level and look how good the GBA was selling in Japan before the NDS came out. In fact, it would´ve sold as good as the PSP in Japan and better than the PSP in the US (don´t forget that the GBA support was slowing down because of the DS - it would´ve sold better if there was no DS of course...)