While I agree the battery should hold out for a very, very long time, I am not sure your assumption here shows the entire picture. True, the battery might still be working decently after 5, 6 years, but in my experience all batteries lose part of their capacity over time. For example, two years ago I used to be able to get almost three hours of usage from my laptop, these days it barely holds out for more than two - and I have been rather careful to always discharge it fully before charging again. Nothing dramatic, but I do expect in three, four years the battery in my reader will have degraded at least enough to make charging a nuisance, not to mention it will probably not last a whole holiday anymore.
Of course, the future is hard to predict, but I did not buy my reader thinking I would be replacing it in two, three years. As such, it is good to know replacement batteries are available
