> Monetary policy is about increasing or decreasing the money supply. The argument is that it cannot have long-term real effects because if it did we would all be incredibly rich

No, even if the acheivable aggregate real expansionary impact of monetary policy were unbounded (which no one argues it is) we wouldn't all be rich because monetary policy also has a distributional impact. But, more to the point, having bounded real impact isn't the same as zero real impact.