As I've said before, the 3DS should be doing BETTER. It's not. Heck, even still, people are buying the 6 year old PSP over the new 3DS. The DS was able to do well because of games like Nintendogs and New Super Mario Bros. Nintendogs is already on the 3DS and DS owners are not upgrading. DS owners do not want a 3DS. There also isn't a 2D Mario in sight. The only saving grace is Mario Kart and Animal Crossing, but if Nintendogs did not help, then why would these games?
The DS had a weak launch for many reasons but mostly due to launching with an N64 game. The system starting doing well when it made games that were like NES and SNES games as well as new games (Brain Age, Nintendogs). The 3DS will soon have OoT which will also not help the system. Time will tell, but DS owners are looking to not upgrade and they will reject the value of 3D. This is more serious then people think because Nintendo's handheld market has been tarnished.
It's way too soon to jump to those conclusion. There are way more factors than 3D that people could be rejecting (price, battery life, not enough games), and it's possible (most likely) that this is just temporary.
Also, how do you know OoT will not help the system? Were you there at launch in the near future?