Whatever is stored in OP_return is visible on the blockchain and everyone will have access to your private keys since it cannot be deleted, making it vulnerable to attack.
Private keys should be store offline for maximum security so that no one can have access to compromise your wallet. This is why offline storage is the best. Hardware wallets and electrum cold storage is the best way to keep your funds safe because the private keys are never connected to the internet. Online storage has many flaws than offline storage.
You forgot to read where he says "
password protected with BIP38", you are not including the raw private key in the OP_return script but the encrypted version, and that means that even if your encrypted private key is exposed to the public, no one can decrypt it since only you know the password to encryption. OP_return can work, it satisfied the conditions above.
There is nothing like a second Bitcoin, there is nothing on the internet that comes close to Bitcoin that is decentralised, easily accessible, and secured like Bitcoin except if you don't want to satisfy the above conditions.