I guess they were really only relevant for maybe 5 years, ~2005-2010. It primed Apple to make the iPhone (which is huge), but it ended up being a bridge technology between CDs and streaming.
Of course, most people haven't had smartphones for more than a decade at this point. (Certainly not much longer on the outside especially if you exclude Blackberries.) Not that smartphones are likely to evaporate anytime soon.
It's an entire product category that swept in and had less than a decade of relavence before getting washed out in the tide.