You are correct. I used to be in the home building industry, and subs paid their employees in cash and didn't treat them as employees. All knds of businesses that receive cash pay cash under report their income. Restaurants are likely the worst.
But still, if your goal is more revenue you need to audit the big money. If you audit the small under reporter you might get more compliance via word of mouth.
It's not 87,000 new auditors. AS Mojo points out the money will be spread out.
The IRS is woefully understaffed in customer service. Also, in the article It is estimated that 50,000 employees will retire in the next 5 years, and this money is spread out over 10 years.