> I'm actually quite confused as to why the people with the oldest devices—whose Kindles presumably had the longest usable life—are the only ones getting free replacements.

Maybe there is a lifetime clause in the AGBs? If they promised 15 years of usable time (for example), this might be their way to get out.

Also, the longest customers are probably the most loyal and bought a lot of books over the years, while also being the smallest subgroup.