While structural factors play an important role in the economy, and they may have a lasting influence, not all economic developments could or should be attributed to them, and the economy can benefit from sensible aggregate demand policies, including monetary policy. These policies can certainly prevent excessive swings in aggregate demand, in both the upward and downward legs of the business cycle.
Policies can still be effective, but they must be formulated, adopted, and implemented with the understanding that structural changes do occur, and when they do, this will change the context and effectiveness of policies.