The tax code is excessively complex to provide loopholes for the wealthy to use to lower their tax burden.
For example, one of the biggest loopholes in the tax code is the carried interest exception. It created modern Silicon Valley; YCombinator exists solely because of this loophole. Most of the nation regards it as the most blatant subsidy in the tax code, but it is essential to startup financing. How would you feel if they got rid of it?
And the "flat rate" tax rate you propose is a huge subsidy to the wealthy, who derive the most benefit from a stable government and therefore should pay the most to continue it. Conversely, any rate high enough to fairly tax the wealthy would excessively tax the poor and middle-class. This is why we have a progressiv (i.e., complicated) rate structure.
Aside from the fact that we might lose HN? I'm completely for that. Most startups are just re-imaginings of existing businesses but worse. Silicon Valley gave us the gig economy and commercial-surveillance-as-business, and has flattened the Internet into like 6 websites all of which copy-cat the hell out of one another to the detriment of all the features that made each of them notable in the first place.
And you know, crazy thought here, but if I don't make enough money to offset my living expenses, I go broke and go into bankruptcy and lose everything I have. Maybe if a business can't exist without ludicrous subsidies and serves no purpose outside of that, maybe we don't need it?