I suspect that stagnating iPad sales are because people don't need to upgrade them as much as they once did.
iPads are exceptionally well built imo and apart from the iPad 3 they appear to last for years. Phones are usually replaced as contracts are renewed (12/24 months), iPads aren't.
I've had my Air 2 since it launched and it's still running like a champ, does everything I need. My grandmother has an Air 1, my partner a Mini 4, mother an iPad 2. The 2 might be slow, but for it's age it's still chugging along.
People don't buy iPads because people don't need to upgrade their iPads.
I agree, iPad's have exceptionally build quality there is no denying that. The issue is that when the iPad was originally released it was mainly used as a consumer device as there were was very little at the time to create anything. Over the years it has improved to create and edit to a limited level, however we are not there yet. iOS for iPad is lacking in this department as well, though I have no issues with the grid layout of icons/apps on the home screen, the spacing just does not make any sense. It resembles a stretched out iPhone home screen. The apps on the other hand are customized to use the additional screen estate to a degree, it still needs work.
I am not sure if even Apple has figured out how to create on the iPad will function. We have word processing, data charts, presentation that are all office related functions. How do we migrate and convert processes such as coding, imaging editing (not novice), music creation, video editing (not novice), etc to the iPad.
We know that Apples ARM chips are powerful, the issues is that most of it is going to waste, so far Apple demonstrates game performance. That may entice some to the platform, however it still does not answer the question of creating.
I prefer the size of the iPad mini, it fits comfortably in one hand, portable enough and has a large enough screen to be productive. I sold it when I purchased the iPhone 6+/6S+ as it served most of my requirements. I still miss the iPad mini though, if it had 3D Touch, Pencil support, Smart Connector support, TrueTone and stereo speakers along with TouchID 2 and a TrueTone flash for the rear camera that would be an instant sale for me as I am thinking of downgrading in phone size.
Plus the hardware is capable to last for 3+ years, at the reduced cost compared to buying a Mac (if not creating) who would not want a lower cost mobile device.