The major reason I end up upgrading is obsolescence. For example, I can now no longer upgrade some apps on my IPad because it can’t accept the newer IOS. It still works OK, but there will be a time when the programs I use most won’t work. That, and being unable to “replace” my battery will get me to replace the whole unit. (Apple doesn’t actually replace the battery. They just charge you $99 for a refurbished model.)
Also every OS upgrade on an iPad takes up more and more hard drive space. I got my current iPad around two years ago. At first I had plenty of hard drive space but now the IPad IOS is taking up over 30% of the hard drive.