Quoted:
Can't be arsed to do the research, but are we talking about a 75% marginal tax rate, or overall income tax rate? Because 75% is clearly insane (well, 50% is clearly insane, but I digress). From what I've read about the "good old days" of US tax code that liberal progressives like to dream about, at one point the top *marginal* US income tax rate was circa 90%. So, above a certain annual income, you were "allowed" to keep 10¢ on the dollar. But the overall effective tax rate of an individual paying this marginal rate wasn't 90% because that's not how marginal rates work (overall was still ridiculously high, though).
Also, NOBOY paid these 90% marginal rates; they were "sucker rates" only for people not clever enough to avoid them through asset allocation etc. Don't know how that compares to the situation in France.
There's that, and there's the fact that it wasn't nearly as easy in 1950 to start an e-business selling an e-book about how to sell e-books and make millions of dollars. You had to make the equivalent of $3 million in today's dollars to max out your tax rate. Even if you get into the top tax bracket, your effective rate will be much lower because most of your income is being taxed at lower rates.
In the case of France, however, you're going to have people who earn $20 million a year, and their effective tax rate will be approximately 70%, before you add on the wealth tax. I calculated earlier that a person worth $300 million, with an income of $20 million, would pay more than $20 million in taxes under that system.