@Thomas,
Easy. It proves that economic output is not independent of economic burdens placed upon the economy, of which tax rates are only one. Some burdens are cyclical, others are artificially placed by legislative action. Burdens may include the cost of energy, the cost of capital, the cost of labor, the cost of raw materials. Then there is the cost of taxes upon business.
Econ0mic burdens affect the economy, that much we have seen proven. Now, I assert that tax rates are a burden on business, and it may affect the price of widgets and thus the number of widgets sold, the profits realized, and the relative financial health of the widget maker, the widget maker stock owners, and the customers of the widgets.
Come on, Thomas, I am surprised you can't figure this out yet.
People that deny the Laffer Curve are essentially denying that economic burdens placed upon businesses have absolutely no effect upon those businesses. Such a belief is nonsensical on its face. As an okie coming onto this forum, arguing with supposedly educated and sophisticated people, I am surprised such a basic fact of common sense is found to be so incomprehensible by the same.