How are you coming to a 3 year life of the Kobo? Given the batteries in most consumer electronics are calculated at between 300-500 charge cycles, IF you charge the Kobo once per week (worst case scenario), that equals 50 times a year. At worst you'd be looking at 6 years before the battery starts to decade (under ideal conditions).
If you take a look at the wiki here on the forums you'll see pics of a Kobo dismantled, so if in as you are saying 3 years the battery needs replacing, you should be able to dismantle the Kobo and replace the battery with relative ease. Provided the same voltage (and at least the same mah to keep good uptime between charges) battery is used you'd be fine.
Then again as others have pointed out with changes in technology etc, it is more probable you would have replaced it with a newer model because you want to rather than the Kobo is all worn out.
Ultimately if you are worried about battery replacement you could look at the Eco Reader, it uses a standard Nokia mobile battery, which is user replaceable.