If DS was a fluke where's the "inevitable" handheld decline?

[Pyro as Bill]

GBA - 81.5m     Software - 377m

PSP - 80.8m     Software - 303m

3DS - 61.6m(Sep'16)  Software - 293m (Sep'16)

PSV - 14.6m     Software - 53.6m

So far this gen: 76.2m     Software 346m

 

3DS (Europe/Japan) has already outsold GBA (Europe/Japan). It's ~20m behind in the US but the PSP was too and 3DS still has some life left yet.

Given the onslaught of smartphone and tablet gaming surely the decline should have set in and handheld software should be much lower than in the GBA days.