It's 'just' a battery. From an engineering standpoint it's just a component that Apple also doesn't produce themselves.

If apple no longer sells the battery, it will be trivial to find a suitable replacement. As long as there is demand, there will be loads of shops able to replace the battery for you.

The real question is: will people who buy $500 branded headphones really have their 5+ year old headphones repaired, or will they but the latest-and-greates shiny new thing?