I don't understand why battery failure is a "when". The only batteries I've ever had fail in rechargeable electronic devices were replaceable packs where water got into the compartment. Perhaps I've just gotten lucky?

Batteries are chemical devices and have a finite lifespan. There's enough confounding variables involved that some people get real superstitious about it.

Through a combination of internal and external factors, lithium batteries go through irreversible chemical changes on every charge cycle. Total charge capacity always trends down. Depending on a large number of variables, this might happen quickly or it might happen so slow you might never notice at all. It's all down to how and where the battery is used because temperature is mostly what drives the chemical reactions.

The other thing is that lithium batteries which have been deeply discharged below safe levels are permanently damaged. Just like charge cycle wear, this can be immediately apparent, or something that goes unnoticed forever, but the cell is damaged and will never be the same. Recharging such a battery can cause physical damage inside the cell.

Point is, batteries are chemical cells. They don't last forever because most chemical reactions are not perfectly reversible. Their specific application and environment strongly affects how long they last. But we do know conclusively that when lithium batteries are kept hot and charged and discharged at high current (such as in a handheld gaming device), they degrade faster. Cells which are kept at a stable temperature and low currents (such as headphones) degrade very slowly and can last a long time.

If you look past superstition, there's an entire industry that's been rigorously studying this problem for decades. There's a lot of literature and evidence.

Lithium cells do degrade. How fast they degrade depends on cell quality and environmental factors. It is, unequivocally, a question of when a battery will fail. It might well be a decade or two, but it will fail eventually.

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.

The Switch 1 is almost 10 years old. Batteries don't have that much longevity.

And it was unnecessarily hard (actually dangerous due to fire risk) to replace it with a 3rd-party battery replacement due to excessive amounts of strong adhesive holding the battery in place.

My PSP, maybe five years ago, had a swollen battery. A friend a couple days ago was complaining that his PS4 controller's battery held no charge at all.

All li-ion batteries degrade over time, they may not fail completely, but the capacity will be diminished for sure.

How many years do you use them?