First of all, those 18% of taxes are coming from different places.
Check this graph from Reuters. Over the past few decades, we've been taking in much less tax revenues from business taxes (corporate, excise) and taking in much more from payroll taxes (social security, Medicare). That goes along with the fact that we cut business taxes and raised payroll taxes during the 80's. So if the Laffer curve exists, it apparently doesn't apply to half the taxes that we collect.
Second, we can talk about wealth disparity. The income tax is a tax that primarily affects high incomes, while low incomes can often deduct out of income tax completely while still paying FICA taxes. You would imagine that during a period like the 00's in which the gains in GDP were mainly being absorbed by high income earners that you would then see a rapid expansion in income tax revenues, but as the chart above showe, income taxes went up during the dot-com boom but barely returned to a historical average near the peak of the next boom. That would indicate that we're going to need another overinflated bubble to get income tax receipts back to the historical average after yet another recent drop.
There's also the fallacy that more tax cuts will continue to drive GDP and employment, while the current trend is that liquidity amongst corporations is substantial enough to create more investment but that hiring is being held back by a lack of demand. While tax cuts may be a driver of employment, it's unlikely as big of a driver as demand, which has sharply dropped as the costs of consumer goods and services has risen. If the deficit is a concern, then income tax increases would likely have a lesser impact on the economy than in cutting services that contribute to the working and middle classes that drive demand.
