The only risk I can think of is that you might run out of battery power sooner then you would expect. As they age batteries will take less charge so their capacity diminishes. However I don't think there is any danger of damaging the camera. Battery voltage doesn't increase and older batteries don't explode.

Canon are probably very conservative in their estimation of "end of useful battery life", just like they are very conservative when they tell you a cartridge has run out of ink!

Batteries life depends on both the battery design and how it's used. For a typical Li-ion cell life is maximized if it's not fully discharged and not overcharged (or even charged to 100% capacity). I don't know if this applies to the Cannon batteries and charger but I assume the charger cuts off early enough to prevent any premature aging of the battery. I'd also guess that the camera shuts down before the battery is truly fully discharged to avoid shortening the life.

Typical numbers for a Li-ion cell might be 1000-1500 cycles of 95% to 50% charge/discharde before capacity drops to about 75% of the original capacity.

My experience is that at the true "end of life", the battery simply won't hold a charge or will refuse to charge. As long as the battery seems to take and hold a charge, I just keep using it (and carry a spare of course!).