Battery failure is a "when" because batteries have a limited number of charge-discharge cycles. Modern lithium-ion batteries have a life expectancy in the range of 300-600 cycles. So if you've never had such a failure, it probably just means you're not a heavy user of your devices.

I try to keep my cell phones as long as I feasibly can. Every single one I've used for more than 3 years has had its battery fail (as expected for a device that sees such heavy cycling). My current phone is on its 3rd replacement battery.