Reasons for decline in revenue is simple their own accessories and competition
We can talk all day about many things, but for me it's simple.
There's a lot of competition out there, so when you bring out a product which renders a good few of your existing accessories useless, you think. What an I buying here? A totally new product with accessories, so I might as well take a look at the other offerings on the market.
It stands to reason that GoPro looses business that way. Some other options are cheaper, come with accessories or are just better devices.
Compatibility example.
1. Housings - As a diver this is important and there was a time when a dive case came with a GoPro this alone made the purchased a no-brainer. Now I have I buy a new case if I want the Hero 5. Not to mention spares for trips.
2. Batteries and chargers - GoPro has always been less than amazing with battery life. So I stacked up on batteries and chargers. With the Hero 5 you'd have to buy new ones and the additional batteries which plugged in to the port on the back, they are now not usable with the Hero 5. There was simply no obvious reasons to do this again! It smacks of bad design or they are doing it on purpose.
3. The loss of the port - this renders lots of items useless.
I get why they wanted to have a screen, however not having the port kills all add-on's. To be fair why not simply have the screen removable like the old one and keep the port. Then you have choice.
4. Form factor, it's changed again, so things like filters won't fit either. Then there's things like carry cases which won't fit, and so on. Simply no need to do this. Why not be clever with the tech design and keep the same size, or at least at the front and all one needs to do is get a new rear door for the housings.
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IMHO GoPro, you've messed up big time.
For me you should have keep the Hero form factor and crammed in all the tech you can and kept the port. Then upgrading "just the camera" would have been a no-brainer as it has been for the past 2/3/3+ and 4 which I've purchased. I'd have purchased other things too over time as they came out or when they broke.
You could have changed the lens, but at least external case mounted filters would continue to work.
You could have kept the same battery size and upgraded the capacity if possible.
You could have kept it at a reasonable price without a dive case, or offered it as "free" and kept the pretty high price.
As for the Session, I'm not sure what you were trying for here, the Hero is small enough. OK, have a cheaper product, but seriously a non-removable battery makes it product doomed for battery failure and a forced purchase.
Finally, essentially you've lost my faith that you'll simply do this again and devalue if I invest again in another Hero 5.
However, by then I'd have probably already purchased a 4K or higher 360 camera, so you might have lost me for a long time. Which is sad.