The thing is they do make great cameras but these days they are so incredibly expensive compared to alternatives.
My dive buddy and I have have a Hero 4 after our 3 broke it's mount and sank - but there's no point in trying to use it on a dive in 4K or with the screen on or without an extra battery as the cold waters will drain the thing in minutes.
Also compare the knock off models you can pick up for 30 or 40 quid that will do just as good a job and that you don't care quite so much if they are lost or damaged and you can see why their sales are slipping.
Even the mid-range brands you can pick up are half the price.
So not really a surprise when they say they didn't sell at the same price as when they launched, but they started to when they dropped the price...