The 3DS' numbers only look good when compared to the Vita's. It is also struggling. The entire dedicated handheld market is struggling...that's the big picture and where the true concern should lie.
Launch aligned, the 3DS is pretty much in line with the sales performance of the GBA and the DS.
While it's true the 3DS hasn't taken off so much in the west yet that's due to a number of factors. Price, as you said yourself, is a huge point. the GBA launched at $99. The DS Lite took off at $129. The entry point for the 3DS and 3DS XL at $169/$199 is simply too high. Another factor is the state of the DS. It may not be seeing major software releases anymore, but the system is still moving a lot of units and is a tremendous value with an amazing software library. As the DS fades out/stops being produced and the 3DS library continues to grow consumers will eventually make the upgrade. Another point is the simple fact that the US is historically slower to adopt new hardware.
The explosive growth of the tablet/smartphone market has certainly had an impact but Nintendo still very much has a place in the market with their handhelds. I think the biggest impact smartphones/tablets have had is in securing the casual market. Their mass market, more casual oriented hits like nintendogs, brain training, crosswords, etc no longer have the same selling power. Casuals are perfectly content playing angry birds and farmville. However, there's still a market for gamers who want a deeper handheld experience, and that's what Nintendo is offering with the 3DS. You're simply not going to find experiences that rival kid icarus, monster hunter, resident evil, kingdom hearts, etc on a phone or tablet.
When all is said and done, will the 3DS meet the 150 million+ units sold by the DS? No, but then again, basically no system will. the DS was a phenomenon and will likely be the best selling game platform
ever before it goes out of production. That doesn't mean the 3DS can't be a success selling "only" 80 million units, which I can easily see it doing.