I would guess you are meant to buy a new one if you manage to use the device so much that the battery actually becomes unusable.
Typically, Lion batteries don't become unusable, they fade away. A new one has 150 CIPA shots claimed, so the question is when does it drop to a life of say 100 shots? Life of a Lion battery is said to be when it reaches 70% capacity. How many would a person need before a 2 hour recharge?
If you don't use it, the battery dies even quicker. Batteries reduce in capacity as a function of recharges (300-500 typical life) and time. So, in 2-3 years, the likelihood of a dead or greatly reduced capacity is to the point that enough will need batteries to make it significant. I have a number of electronic gadgets that need new batteries after 3 years. I would not buy a device that needed replacement after 3 years or even 5 just because the battery was dead. There must be a way to replace it. Likely, it will be a $100 charge from Canon. Maybe I should sell every 2 years and get a new one? My credit card offers a additional year of warranty so I'm covered for 2 years.
I wear a Cochlear implant which uses Li-on batteries. They fully discharge and must be recharged every day. They fit demonstrate the validity of the 300 recharge cycle range for smaller batteries nicely, I need a new one every year. They will keep going for much longer, but the capacity continues to drop so eventually a charge just lasts just 2 or 3 hours and that is impractical. The battery in the Canon device is not going to be fully discharged and recharged every day, so 500 charges might take 10-50 years if it weren't for the tendency to fail over time as well. I'd bet that around 25% will need batteries in 5 years. By then, of course, the value of the camera may very well be less than the cost to repair it.
There is a very good article about life of Li-on batteries here:
Discover what causes Li-ion to age and what the battery user can do to prolong its life
batteryuniversity.com