Suppose evasion and avoidance were completely eliminated. Won't happen, but work with me here. What is the total estimated impact of that, in dollar and percentage terms?
I'm guessing it would be less than a 10% increase in tax revenues. Though I'm willing to believe I'm off by a lot. As much as people scream about earmarks, they represent a trivial part of spending. I'm assuming this is similar.
Just as a thought experiment, suppose increasing tax rates leads to increased avoidance. But suppose the total amount collected increases. Would that be a good outcome? Sure, it would shift some of the burden away from aggressive avoiders, but there's no rule you can pass that some people won't try to exploit.
Closing loopholes is good and should be done regardless of anything else, if only to reduce the cost of compliance for everyone. (Simpler rules are easier to comply with. Loopholes == complexity.)
But compared to doing only that, what percentage increase in income tax rates would be required to generate equivalent tax receipts?