The media has been filled with stories on falling iPad, Android, and Windows tablet sales. Here is one of the reasons why.
The iPad, at least starting with introduction of the iPad 2 in 2011, is so well designed and lasts so long that many people see no reason to upgrade to a newer model. This Macworld article by Joel Mathis explains why he remains happy with his 5 year old iPad 2.
I must be a bit older than Joel, and with worse eyes. I had an iPad 2 and loved it. But when the Retina screen iPads came out, I could tell how much better the display was for my aging eyes. I skipped the third generation iPad (in my view, the only possible "dud" in the iPad line) and then purchased from the Apple outlet store a refurbished 4th generation iPad (the first one with the Lightning Connector and the last one before the iPad Air) with a Verizon data connection that I could add to my Verizon Wireless account for just $10 per month. Tests showed that the 4th generation iPad was twice as fast at some functions than its predecessor. It has always been plenty fast for me.