One more vote for ‘they will last until they die’. We have people who have their aids for 10 years, yes they upgraded for new features but old ones are still working and serve as a backups.
However, if aids are rechargeable, then battery will probably die of sitting unused for years.
But until you smash it with a hammer, your dog or cat chews it, you drop it in the sea, there’s no reason for them to ‘just die’. Extensive sweat could be the culprit for circuitry death though.
Yes, receiver part can die, and if you wear classic BTE, it might not be repairable after some point eg no spare parts, but that’s the same with RIC aids and their receivers.
So basically as long as there are spare parts for them (somewhere, not necessarily with the manufacturer) and circuitry works, they will work.
What is more probable is that after 3-6 years you’ll be the one to say - I want new ones because of tech, or new color, or if your loss deteriorates so much that you need different form.
Or you’ll lose them and you’ll need a new pair.
But they’re not made to die on 4 year mark, or any such arbitrary number.
Interesting is how different fitters say different things, those oriented to selling were the ones that would mention similar arbitrary mark to me, but I bought mine from two who said what I said above - if nothing bad happens to their circuitry we can keep them working as long as there are spare parts (battery door, receiver).