You're right. All that needs to be done in that case is to simply challenge the laws written under that usurped power in court as unconstitutional.
Oops. Wait. That was done, and the courts have found that the power was NOT usurped.
Now what?
Oops. Wait. They couldn't do that so the courts "found" the power someplace.
Such usurpation will continue until there is a police state (followed by a civil war) or a court will actually read the constitution.