OK but you'd still be able to use the open source "password manager" to export the keys - which solves the issue lapcat raised in this thread - even if relying parties blocked it for authentication, which would be a separate issue.
Someone could develop a "passkey export tool" purely for the purpose of doing credential exchange then local export.
Or are you saying the credential exchange process itself could block providers?
You misunderstood lapcat I think. They wanted Passkeys stored locally exclusively. And they wanted to be able to use them. The issues are not separate.