Can a 51% Attack Permanently Alter a Coin’s Underlying Protocol or Smart Contracts?

A 51% attack cannot permanently alter the underlying protocol or the code of smart contracts themselves. The attacker can only manipulate the transaction history, censoring transactions or executing double-spends.

To change the protocol, a majority of the network nodes and developers would need to agree to a hard fork. The attack's damage is primarily financial and reputational, not a permanent code change.