That's the right question to ask. The answer is no, because new security vulnerabilities are disclosed every hour. A large organisation will implement layered

That's the right question to ask. The answer is no, because new security vulnerabilities are disclosed every hour. A large organisation will implement layered security (otherwise known as layers of the onion) to prevent this type of attack. This means; more secure passwords to access the password database, fewer people with access, rotation of access passwords, auditing of backup storage and encryption, etc etc. Clearly Yahoo's layers of security were all broken to allow this type of theft.