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.
If I were a betting man, I'd put a huge bet that this is most of the reason. iPads are not like iPhones. Apple rolls out a new iPhone or even an "S" and just about everyone feels like they simply MUST have it (why? I don't know but it is a game we all seem to be playing). iPads just keep right on "chugging along."
I almost found myself finally replacing my iPad Mini 2 yesterday. Why? Because I mistakenly left it behind at a restaurant and thought it might be lost. Fortunately, I noticed pretty quickly and rushed back there. And they had it to give back to me!
I suspect the biggest reason to "upgrade" for many is that the old one has stopped working, been damaged or been lost. Else, newer ones don't seem to offer all that much more than older ones.
Same with Macs. Why don't we all upgrade our Macs with every new generation? Because existing Macs seem to do just about everything that newer Macs can do. And these days, embracing "the future" in Macs often seems to mean complicating your life by needing dongles, etc to be able to connect it to the rest of the tech you already have.
Is this a problem? Only a problem for those worried about how much money Apple makes. For consumers, if what you have works "well enough," you get to keep your hard-earned money in your pocket instead of giving it away to buy the new one that is almost the same as the old one. More simply, you still get the bulk of the utility of owning (albeit older than newest) one AND you get to keep your money too. How is that a problem for any of us to worry about?
When do I replace my Mini? Probably when it's damaged, stolen, lost or maybe a new Mini is released that does something so different & important to me that I simply must have it. Haven't seen that kind of innovation since I purchased this one. Is that a problem? Not at all for me. I actually like to get a lot of utility- years and years of it- out of tech hardware I purchase. Basically, Apple built a great tablet... so great IMO, it doesn't need to be replaced every year or two... or three. Nothing wrong with that- I wish more products were that way.