The approach you describe is one that we use where we regulate a sector. They have to achieve a certain standard. In doing that, we can reward companies or farmers for doing better than average. An offset is something that you're not required to do.
The challenge we have with offsets is that there is no regulated requirement. We're giving somebody an economic benefit. As a matter of policy, the government has decided, consistent with basic international practice on offsets, that offsets should only recognize changed behaviour going forward and not be a reward for early action.