A few other comments:
Unfortunately, we don't seem to have frequency of repair data for eReaders. If we did, your Kobo N647 might be far from the best. Or it might be the best. I don't know.
With Kindle eInk readers, I can safely say that the main cause of failure is impact screen damage. If their screen was the same size as your calculator, maybe they would (with battery replacement) last that long. A few of them, anyway.