The 3D effect has helped, but without it, Nintendo would likely have been able to launch the system at $200 instead of $250, and we'd have the system for no more than $150 for a standard model by now. And although the 3D effect can make a game really work, more often than not a game can either forgo 3D or focus on an aspect that does not require it. Remember, the best selling game on the 3DS, Pokemon X/Y, can only use 3D some of the time.
Without the 3D effect, I think the 3DS would have had a better launch, but would have slowed down a bit around late 2011 before picking up speed again. It would today be at around 85% of IRL sales.
The 3DS wouldn't have games like:
Kid Icarus: Uprising
Kingdom Hearts 3D
Resident Evil Mercenaries 3D
Resident evil Revelations
King of Pirates (trilogy)
Mega Man Legends 3 (canceled)
Paper Mario Sticker Star
Indie games like Jett Rocket 2 & Mutant Mudds & etc. etc. etc.
Dillon's rolling Western & Sakura Samurai & Ketzal's Corridors & the like wouldn't even exist.
Monster Hunter 4 (never coming to Wii U)
And many other exclusive titles. (but still not all of them)