Of course the 3DS's games libary isn't as impressive as the GBA's. The 3DS is only a couple years old, so it hasn't had a whole lot of games developed for it. The 3DS is only, roughly, two and a half years old and the GBA was in production for 7 years so... Comparatively though, the 3DS is doing well - Just over 30 million units sold so far, compared to the GBA's total of 81 million units. As it is right now, the 3DS is close to hitting the same amount of sells in Japan as the GBA did. Additionally, the base model 3DS has sold more in Japan when compared to the DS's base model.
So realistically, the 3DS is doing just fine.
Honestly though, I see the entire handheld market collapsing soon. The handheld market is mainly aimed at those seeking portable gaming and tablet's can do that, perhaps even better thanks to their superior hardware.