I don't see this ever being relevant on the scale of plastic pollution; people will buy a few and use them for years. For example, is the world being overwhelmed with smartphone cases?

You can create the holder from any biodegradable materials, leather or cardboard etc. No need to use "non-recyclable plastic".

But that’s precisely what all these Chinese marketplaces won’t offer.

No one is forcing you to buy the holder from China.

You can go to your local artisan and ask them to craft it from any material you choose. It's not a complicated form to create a slot for.