[3np]

Arbitrage trades and related activities like MakerDAO keepers would be legitimate transactions vulnerable to this (essentially someone else extracting the value from their discovery). Granted that is a very small subset of users.

[nonsens3]

Liquidation contracts and Arbitrage contracts do check the caller and would not allow to be executed by non-approved senders. This raises the bar, so that you can front run only contracts that you can implement and deploy.

If anyone could just replace an address and execute a profitable transaction by being first on existing contracts, surely miners would be doing it already, no?

[3np]

> If anyone could just replace an address and execute a profitable transaction by being first on existing contracts, surely miners would be doing it already, no?

To a large degree not yet.