I have an 8 year-old cell phone with an original Li-Ion battery that is about the same size as the battery in my e280 and it is still going strong! This phone gets use every day, much more than I would think an mp3 player gets.
I believe your estimates on the life expectancy of these little guys are too critical. The Li-Poly batteries in the “disposable” units (FUZE & CLIP) are designed to last longer than the Li-Ion ones. You could be looking at 8 - 10 years before they “bite the dust!”
