Economic theories, like scientific theories, are successful if they correctly predict what will happen if you do X.

In addition, an economic theory can only be useful if the actions that they dictate can actually be put into practice; if nobody's going to follow what the theory tells you to do, then it's of little use at all.

This is the big problem with both MMT and Keynesianism: they're both great figleafs for politicians to wear when they want to spend in order to pander. But when the theory tells them that the situation has changed and they need to change their actions accordingly, they don't heed the need to raise taxes (MMT) or slash spending (Keynes).

Unless we really do the thing, then pointing at a given macroeconomic theory is just an excuse.

> In addition, an economic theory can only be useful if the actions that they dictate can actually be put into practice; if nobody's going to follow what the theory tells you to do, then it's of little use at all.

That is not a requirement for a theory to be useful. The fact that not using it has a cost is proof of its utility.