http://www.vgchartz.com/yearly.php/?reg=World&console=&date=2001&maker=

This shows that 3DS & GBA are tracking very similar globally.

GBA end of 2001-13.12 vs 3DS end of 2011-13.25

GBA end of 2002-28.01 vs 3DS end of 2012-27.36

GBA end of 2003-46.72 vs 3DS end of 2013-42.20

GBA end of 2004-62.91 vs 3DS end of 2014-??? Likely 55ish

When u add in Vita sales it looks like the handheld market has reverted back to gen 6 levels which isnt bad at all, 80 million is still a sizeable market. Will it continue to shrink next gen or is 80m where it will level out at? Thats impossible to tell as of now.

Ur right that GBA didnt have the mega hits like the one u listed for 3DS but ur def selling GBA short. GBA was the nostalgia device, Super Mario Advance, Classic NES Series, Pacman Collection, Namco Museum, Donkey Kong Country, Link to the Past, games that havent been released in 10-20 years and before the times of emularion and virtual console, also the only platform at the time supporting 2D gaming. Also the first Mario Kart on handhelds, the long awaited return of Metroid, return of 2D Sonic, introduction of Mario & Luigi, Golden Sun and Fire Emblem/Advance Wars in the west. Pokemon, Yugioh, Dragonball Z were all big hits at the time, Spyro/Crash on handhelds for the first time, sequels to Kirby/Wario Land.

Outside of Pokemon, GBA didnt have any single megaton hits but it made up for it with an excellent library for a cheap price. DS/PSP greatly expanded the market with casual hits like Nintendogs/Brain Training on DS and multimedia/3D gaming for the first time on handhelds with PSP. Things like that are easily accessible now on smartphones/tablets the expanded market is gone and its very likely we simply see the handheld market shrink back down to gen 6 levels.

The GBA sold 80 million LTD despite having its generation cut fairly short, it almost certainly would've topped 100 million otherwise.

The 3DS will probably stall out at 75-80 million, that's how its looking right now. That's pretty decent, but it's also lower than what Nintendo's sold since like the 1990s.

The lack of year over year momentum for the 3DS indicates smartphones/tablets are spreading like a cancer more and more every year, Nintendo is fighting like crazy to try and ward it off, but 2014 is going to be the year where their lineup is a little weaker because they have to give the Wii U some support. Tough situation for Nintendo.

I think what's happening is every time Apple or one of the big Android manufacturers (Samsung) releases a big new phone or tablet, they end up getting some new customers each time, and those tablets/phones even if they belong to parents are ending up in the hands of their kids too. 

It's like Nintendo is on a boat, they plug one hole but as soon as they do another two pop up. That's why the 3DS doesn't look like it has any momentuem even with things like Animal Crossing, Monster Hunter 4, Pokemon X/Y, 2DS release, etc. which in the past probably would have created larger increases for them. Now they need this just to tread water.