Then you disproportionately tax the poor since more of their earnings are going to consumption taxes such as sales tax and fuel tax. You also aren't solving the problem that money begets more opportunities to make even more money and that over time a huge income disparity appears and class migration stagnates. Which is where the United States finds itself currently.

Many people point to the 70% tax rate as the ideal because that's what we had in the "good old days" in the 1950s. That's not a good reason to choose that figure either. There were other factors contributing to the 1950s being the "good old days."

That's why finding an "ideal" tax rate is a challenging problem.