Businesses that evade tax would be shut down in an ideal world. Obviously it's not an ideal world and to do so would only make the situation worse and cause mass unemployment.
Tax is pretty low as it is. I'd prefer a slight cut in income tax to boost AD, but seriously people need to understand that not all tax money goes on the unemployed, immigrants, "baby making machines" and the EU. It mostly is used to fund necessities like our armed forces, education, healthcare and yes social security for those unable to work or looking for a job(which isn't easy).
And yes, the shadow economy is a real drain on resources.