But it is the same argument just at different points on the curve. Using the curve to argue against a tax raise just implies you are on the optimal point [for r

But it is the same argument just at different points on the curve. Using the curve to argue against a tax raise just implies you are on the optimal point [for raising revenue] or somewhere to the right and arguing for a tax cut implies you are somewhere to the right. Notice both arguments have similar assumptions and the only difference is the unlikely possibility you are at the optimal point. Of course one of those arguments is stronger because it does include this unlikely possibility but not much stronger. I think the main argument is whether you are to the left or the right of the curve not whether you are at the optimal point. Also, I’m assuming it’s unlikely the government has set the tax rate at the optimal rate to maximise revenue. But this assumption is probably dubious because even without explicit knowledge of the curve governments are probably pushed towards the optimal rate.