How many people are doing a spring cleaning of unused passkeys in their password managers? We're talking like a kilobyte of data, nobody needs to delete these things in any kind of normal circumstance.
Sure, it would be great if users would store 5 copies of their encryption keys, with one in a lockbox on the bottom of the ocean. But that's just not going to happen at any kind of scale, so an automatic way of putting encryption keys in a replicated password manager makes sense. And compared to how people normally handle end-to-end encryption keys, it's going to result in a lot less loss data in practice.
I thought the point of passkey security is that you don't have to send the private key around, it can stay on your device. Different passkey per device. Lose or destroy a device, delete that passkey and move on.
I don't know about spring cleaning, but it's pretty easy to delete by accident if you connect to the browser or OS when setting up instead of the password manager.
That said, I've been assuming I could have multiple passkeys per site and that's turning out to not always be something websites behave sanely about.