There is no such thing as an idiot proof flat tax.
Businesses by their nature have very complicated taxes. We'll let them write off a $45,000 truck to deliver product but not a $45,000 mercedes (unless you are in the limo business in which case, you might be able to after all).
Wealthy people, by their nature, have very complicated taxes. Is this a business trip or a holiday? Is this a business lunch or a personal lunch?
We can reduce the loopholes (temporarily) but corporate bought representatives will put them right back in. The flat tax by it's nature is either regressive OR has a massive deduction for everyone which means many of the poorest won't be paying taxes (just like now).
Each share of the national spending last year was $10,000 for every baby, child, senior, and working person. It's about $20,000 if you restrict it to adults who have earnings. That means- unless people can earn well over $20,000 there is not point in working under a totally flat tax. Which means it must be progressive (you have to take money from those who have money to pay).
AND it ignores state and local taxes which are currently higher on the poor than the middle class and higher on the middle class than on the wealthy in every single state. In some states, it's 12.9% for the poorest but under 1% for the wealthiest.