Somebody will make a few good 2d games at some point in the future that sell alot of games and then the other developers will come to rip them off and do it also.
Whatever gets old and forgotten will eventual be new and different after a decade or so (works with clothing, music, and games).
It takes alot of power to properly do a 3d game on a handheld unit (unless its real expensive).
I dont think the GBA will go away just like the psx2 didnt shrivel up and die when a more powerfull console (xbox) came out. The reason is a very large game library.
Now when nintendo tries to make a new handheld that isnt compatible to the current model then things could change.