Well... I believe smartphone ones are becoming more popular (meaning more users) because they not only fully serve the purpose but can also do much more even when the GPS application screen is on (as we all know).
However, the most users on smartphones doesn't mean the GPS devices (dedicated ones) are in decadence; that's only because nowadays a lot of people uses smartphones and it's relatively easy to install and useful having GPS navigation and other GPS utilities on it.
This are only my quick 2 cents on the interesting question.