If 91 million people already have smartphones (I don't know if that is true, I'm taking the linked article at face value), shouldn't we expect growth to be slowing?

There are 310 million people who live in the US and not everyone in that 310 million is a target customer for a smartphone (young kids, older folks, people who just don't like cellphones).

Based on nothing but common sense it seems fairly obvious to me that growth will continue but at a slowing pace -- the market isn't saturated, but much of the low-hanging fruit is snatched up.