This we do not know what the upgrade cycle is a nice angle and probably true. And made heard over and over by Horace, Rene Richie and Jim Dalrymble. It has some truth about it. But it is not the whole truth.
One datapoint that cannot be ignored is app store. App development for iPad is in practice dead. iPad gets some new apps that are ported from iPhone. In last 18 months there has been not single app that somehow take advantage from strengths of iPads. Instead some apps has made simpler because they were ported to iPhone. There never were expectations that iPad pro would be a hit that would turn the numbers around but we should have seen the change in app store and in betas for apps. There is not a sound for change.
App store is the strongest signal that iPad is struggling. It is the flag which indicates the problem. Apple is aware that iPad has potential, it could be the future. But they are not sure what it suppose to be. Marketing is all over the places. They cannot tell the story for use of iPad, although the new apple rarely can tell the story for any product, they are more normal business in that regard also.
And the normal users are also confused. People outside IT-bubble are interested for iPad and buy them. But they struggling to find normal use cases for it. Tech bloggers seem to think that power user is a person that writes up to 10000 words and is confused with spreadsheet and has up to dozen emails. And every one else is like their retired mother. I do think that the average people is working outside home, having their calendars full, lots of emails, writing something with structure to others and balancing their own economy with spreadsheets, Sending documents with email and taking a whole lot of pictures on vacation and holidays. iPad do not work very well for these people. Many features are in fact beast at demos and for those "power users" with empty inboxes in email. When you increase number of documents, emails, calendars etc to the numbers average people use the use for iPad comes a nightmare.
Ipad is a promise of future with the form factor that would work for most people. But it is a future lost because the operating system is locked down to be a little brother to phone.
Ipad is not dead. But it will not be for everyone. It will not gain ground significantly, that ship has sailed. Ironically windows 2-in-1 maybe on to something. Or the phone that transforms to desktop. Phones are something that we all have and carry around. If the identity would be within phone, identification would be seamless by watch (band) and phone. Data would be in the cloud. And wirelessly you could use the apps and data with mouse, keyboard and large display if the work you are doing requires to do so.
Laws of nature are clear, entropy rises. There will be more display sizes. There will not be one form factor to rule them all. iPad is not dead. But they will not kill laptops or desktops either. And they will not kill windows it seems. There will be more of everything. Well maybe smartphones are more significant than others.
iPad usage will stay on this level for the visible future or increase marginally. But because the use cases are not significant people do not upgrade them often so the numbers sold will be declining still. I doubt that apple is apple to change that because it would mean that they would have to make drastic changed in practice and in the philosophy. And apple is best when it is tinkering something existing to the maximum, they are struggling when they are on uncharted territory.