But, if you look at the newest Garmin's, they basically are smartphones 

Technologies are definitely merging but for what we (cachers) use a GPS for, smartphones don't cut it but for most muggles, GPS on a smartphone is great.

You can see this in the compact camera arena, how long will they last as smartphone cameras get better.

It might be that a unit's primary function is first designed and then a phone bolted on so you get 'smartphones' designed for a primary function which it would do as well as the stand alone unit but with the benefits of other technologies bolted on. This could well mean that a GPS smartphone is bulkier than others and is designed to save power, change power source easily, designed to take knocks and is waterproof