I think we have different understandings of what it means o…

I think we have different understandings of what it means or what the purpose is of accumulating 51% of a network's hashpower.
51% attacks don't actually exist; what exist are blockchain diffusions. Let's suppose for a moment that the "donkey" is a rational economic entity that wants to maximize profits and has more than 50% of the network's computing power. At that point, the "donkey" will have to consider what is more profitable for him. He has two options: the first is to continue contributing to the network, and their profits are backed by the price of the satoshis of the network they support, the fees they accumulate from new block subsidies, and the transaction fees they help validate.
His second option is to create a diffusion with a new token. Transactions in already validated blocks cannot be rewritten because other nodes will immediately notice the change in the block's hash. All nodes have a copy of the block headers, even if they lack the necessary hashpower concentration. The only transactions that can be rewritten are those that haven't been confirmed in the current block.