I don’t think iPads die. They do become obsolete for some tasks, and they’ll never be obsolete for others. Keep an iPad on an early enough iOS version, and you’ll be able to use it, for example, to read and watch videos for a very, very long time. So, content consumption in general.
If kept on an early iOS version, the battery is highly unlikely to be an issue, capacity is too high and the software is too efficient for it to have any issues at all. You’re more likely to encounter software obsolescence, websites breaking and lack of app support, rather than hardware issues.
My nearly 7-year-old 9.7-inch iPad Pro works flawlessly for content consumption, and it runs iOS 12, flawlessly. Battery life is very good when compared to updated 1st-gen iPad Pros (your account of which confirms what I’ve read everywhere, it seems to be at around 5 hours of screen-on time), as I am getting 10-11 hours of use without much of a problem, and it is still fast and snappy, barring a very occasional, negligible keyboard lag (which wasn’t present before it was forced out of iOS 9).
Like the vast majority of these discussions put into evidence, iPads’ longevity depends exclusively on user requirements and expectations. You need it for content consumption? Never update and it’ll last forever, barring web browsing issues which you’ll have to circumvent with other devices. You need the latest apps? You’ll update, and eventually the experience will be significantly degraded; so much so, that you’ll likely want to upgrade. If you’re extremely tolerant to this, then it will last even longer. As you can see, there’s no one-size-fits-all answer for this.