It's important to keep in mind that the GBA sold well over 15 million, and possibly over 20 million, after the DS launched. (The DS launched end 2004, the GBA shipped 14.7 million in FY06, FY07, FY08, FY09 and FY10 combined. However, it also shipped 15.4 million in the FY the DS launched, which means it almost definitely sold 5 million in that FY, after the DS launched. Probably far more)
Similarly, the DS has sold over 10 million after the 3DS launched.
If we imagine the 3DS successor launches on January 1st 2016, then the 3DS is likely to be at roughly 70 million sold (assuming 14 million sold in 2014 and 2015, which seems likely). A postlife similar to the DS will then put it roughly equal with the GBA in total sales.
If the successor releases later in the year, then the 3DS is likely to see higher sales than the GBA.
My bet would be that the 3DS will be below the GBA when the successor releases, but above the GBA when the 3DS has finished selling.
