Handhelds aren't the only thing that are declining.
Consoles for 2007 Feb
Nintendo DS – 485K
Nintendo Wii – 335K
Sony PS2 – 295K
Microsoft Xbox 360 – 228K
Sony PSP – 176K
Nintendo GBA – 136K
Sony PS3 – 127K
Nintendo GameCube – 24K
PS4 or Xb one(Still a stupid name to me) can't even match the PS2 at this point and don't even talk about the wii.
Vita is doing dieing GC numbers and 3DS is losing to PSP. Handhelds could just be ahead of a trend that all video games consoles will soon head to.
Fun note: GBA outsold PS3 O.o ok I had to do that