progressive income tax is a politically sound strategy
Politically sound strategies are relative. Talk of a flat tax often rouses popular sympathy. I'd say supporting a flat tax has been a politically sound strategy for a lot of people.
it allows politicians to buy the votes of the many with the wealth of a minority voting bloc.
Taking too much money from the wealthy would result in economic implosion. Economic implosions result in conditions hazardous to a political career (see Tea Party), so politicians have strong incentives to avoid them.
Furthermore, the lower income brackets are spread unevenly across geo-political lines and the ideological spectrum; regardless of dependency on welfare, they don't aggregate into a cohesive voting bloc that supports Democrats and opposes Republicans, partially because the Republicans aren't really a serious threat to welfare (in many historical cases post-New Deal, the opposite has been true).
it also gave congress a ton of power.
Certainly, to meet the domestic and foreign challenges of the 20th century, including two world wars, nuclear power, the Cold War, globalization, and the Information Age. The administration of these events required vast sums of capital; more than a flat tax could have provided.
so your claim is without any rational basis
It's pretty intuitive. The wealthy don't create jobs that don't have a high probability of making a profit, they don't raise salaries when they have a global economy as a labor resource, and, in an economy like that, bonuses are becoming a quaint notion. Ironically, much like any policy that empowers a few at the expense of the many, the prosperity these practices create is a fragile, artificial thing. Rapid industrialization and agitation for more rights in foreign countries (see contemporary Middle East as an example) will lead to increasingly chaotic local politics, endangering the stability of the global economy, and, spurred by their citizens, foreign governments will become increasingly protectionist (which, in comparison to the United States, they already are -- but they will get worse). They will not allow American companies to provide products or services to their populations (aka, closed markets), meaning that America's wealthy will be restricted solely to their country for consumers. Problem is, by that time the middle class will have shrank to a size that no longer supports economies anywhere near the size or productivity of what we currently enjoy.
Consequently, the wealthy, though they will probably remain wealthy, will certainly enjoy far less capital (relative to what they possess now), and certainly far less than if they had been more cooperative in securing America's collective economic well being.