I heard that in Japan, a couple GBA games were advertised as being playable on the DS and/or put right in the DS sections, even going so far as having DS-like packaging, and that was theoretically why they sold better than they might have.
I remember that FF V Advance was advertised with a Japanese woman playing it on a DS Lite.
FF VI Advance came out, didn't it? Chrono Trigger Advance still could. Though, yeah, if they're going to port it to a handheld, the DS is still a better choice than the GBA. I was already saying that about Mother 3 (J), and even earlier.
If handhelds had the five-year "lifespan" of support like Nintendo consoles, all major GBA releases in late 2006 really should've been on the DS. And since the DS has been around for a couple years already, that should have "obsoleted" the GBA much quicker.