They will just replace your AirPods. Not actually replace the batteries. iFixIt[1] had to physically cut them open to get to the battery, there's no way Apple is actually replacing the battery as opposed to recycling what parts they can and tossing the rest when they come in for service.

[1]https://www.ifixit.com/Teardown/AirPods+Teardown/75578

Interesting, I didn't know that. But if you look at it purely from a cost perspective, it's at least a lot cheaper have the airpods swapped for 2x $49 when the battery dies, compared to buying a new pair with case for $199.

Obviously replaceable batteries would be even better. How do other brands of wireless earbuds of the same size fare in this regard?

The wording on that page shows replacing the batteries mean they give you an entire new Airpod (or 2). They say "replacement airpod" instead of "repaired airpod".

Great for the environment...

I see everyone bringing this up, but what if they’re taking them back to their shop, repairing them and selling them as refurbished?

And if they "replaced" the battery -- the thing that causes the most environmental harm???

The electronics and plastic casing are in working order, no? How much did producing those cost the environment (materials, energy, waste by-products), not throwing them away would make sense.