Imagine that an attacker has access to advanced technology, such as a quantum computer, and can crack SHA-256 in minutes:

He identifies wallets with large publicly visible balances on the blockchain.

He uses the SHA-256 crack to derive the private keys.

He transfers the funds to wallets under his control.

At the same time, he attacks miners and manipulates consensus to control the blockchain.

In a short space of time, Bitcoin would lose all security and reliability.$BTC