In the coming decades, federal spending will need to grow just to enable the government to continue to provide the services it does today. One important weakness in the tax system that funds this spending is insufficient tax compliance: In 2020 the IRS will fail to collect more than $630 billion, or nearly 15 percent of tax liabilities. Illegal tax evasion generates unfair differences in tax payments across otherwise similar individuals and firms.
Sarin, Natasha, Lawrence Summers, and Joe Kupferberg. "Tax Reform for Progressivity: A Pragmatic Approach." The Hamilton Project, January 2020.