I'm pretty surprised that the Kindle 1st Generation is from 2007. It's pretty cool that Amazon is still supporting a product from 9 years ago. I would have expected support to drop off or at least not issue any new updates period.
Throwing a few engineering headcount at keeping updates going as needed is more than worth it, and a more exact "when do we stop supporting this" trade-off can easily be calculated against usage and purchasing data since they have vertical integration to that degree.
"The number of people who read [e-books] primarily on phones has risen to 14% in the first quarter of 2015 from 9% in 2012."
I can't find more recent statistics but I expect it's continuing to rise. Convenience always beats quality, eg. MP3 vs CDs, early LCDs vs CRTs.