Is this correct? Wouldn't they still need all of the leaf private keys to decrypt things?
My understanding was that having a CA's private key just enables someone to issue new child keys for that CA. That vulnerability could be addressed with certificate pinning.
Not quite sure what you mean, but for the record, as a general rule CAs do not generate keys. They just sign the public keys coming in as Certificate Signing Requests. Without ever seeing the accompanied private key.
My understanding was that having a CA's private key just enables someone to issue new child keys for that CA. That vulnerability could be addressed with certificate pinning.