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Information security systems are based on rules, technologies, security protocols, and cryptography. The core of information security systems is cryptography. All modern symmetric cryptography is built on the same principle: the encryption scheme is determined by the key. And even if the cryptography itself is "conditionally secure" or absolutely secure (absolutely strong Vernam ciphers), the fact of having a key will always be a natural vulnerability in any security system. First of all, attacks will be aimed at keys (passwords), the "human factor" will be exploited most successfully.
It is this factor that instantly and irrevocably weakens to zero any most secure cryptographic system and consequently the security system in general. There will be fatal consequences if the fact of compromising key or password information remains a secret to the attacker for a long time. The same danger will be acute for any new cryptography that will exist in the era of quantum computers, for any newest cyber defense system of tomorrow.