That is contradicted by the huge number of middle-class tax loopholes out there for, e.g. mortgage interest, small business credits, etc. Several middle-class tax expenditures cost $100 billion or more: http://taxfoundation.org/blog/12-largest-individual-tax-expe.... Health insurance deduction: $212 billion; retirement savings deduction $176 billion; mortgage interest deduction: $101 billion.
Tax evasion using overseas havens is estimated to cost $100 billion per year: http://freebeacon.com/issues/report-tax-evasion-avoidance-co.... Legal corporate tax avoidance costs about the same: http://www.theatlantic.com/business/archive/2016/04/corporat....
> If we instituted an overnight 9%, 9%, 9% "Simcity Tax" system
Sophisticated tax evasion has little to do with claiming various deductions and credits. It is a mechanical and straightforward process to run through those. The real complexity is intrinsic: what's an appropriate depreciation schedule for this asset? Does appreciation of this options contract result in ordinary income or capital gains? Is this payment between related companies legitimate, or a way to shift profits? You can't get rid of these basic distinctions, but at the same time those are the bread-and-butter of sophisticated tax planning.
There is a way to limit tax evasion from companies: stop taxing corporations.