I can understand and appreciate both sides to the argument here - Apple Watch vs dedicated running watch.
I've used several Garmin watches and recently a TomTom multisport watch before getting an Apple Watch. The Apple Watch definitely isn't perfect for dedicated running, but man, once I was wearing it I had a tough time using the clunkier watches from Garmin or TomTom. No where near the comfort of the Apple Watch. I did a lot of running with both watches, then eventually just started to use the Apple Watch full time for running once OS 2.0 came out. I've used a mix of Runkeeper and the built in Workouts app. I'm quite happy with the Workout App, the display looks good, just wish I could have 4 data points instead of 3 visible at a time. And of course, the need to carry a phone for longer runs when I want an accurate measure of distance.
Anyway, just my guess, but I feel over the next 5 years this could play out similarly to what happened to stand alone GPS devices once smartphones became more common. I wouldn't mind seeing TomTom or Garmin make some solid apps for running/cycling/swimming/etc as the Watch hardware improves.