> If they had a 99% marginal tax rate once they have assets of $500M (on any form of gaining wealth), would we have a weaker Microsoft or Facebook or Google doing less good for the world because they wouldn't be working as hard? Would the good done to people on the lower end of the scale not outweigh any possible harm here? That seems hard to imagine.
Maybe you're imagining it on the wrong time scale.
If you say Larry Page can "only" make an effective maximum of $500M, that isn't going to stop him founding Google, but what does it do once he actually has $500M? He owns shares in his company that have grown to be worth $40B so then he owes the government ~$39.5B in taxes. So he effectively has to sell his company to pay his taxes. (And if he doesn't have to pay the tax until he sells the shares then he never will and your tax plan fails.)
Before the tax he was going to keep most of the shares and thereby have control of the company. The primary difference caused is therefore that more large corporations will be publicly rather than privately controlled. So the assumption has to be that that will somehow lead to higher incomes and standard of living for the middle class.
But that doesn't make any sense. Publicly traded companies have no soul. The CEO's job in a publicly traded company is "maximize shareholder value" and that means grinding competitors' bones into dusty by squeezing every cent out of your customers, suppliers and employees. The people being squeezed are the middle class.
At least if the company is controlled by a human person you can have Henry Ford saying his employees should get paid enough to afford his products. A company's owner is the only one who is authorized to decide that there is something more important than shareholder value.
The problem with today's economy is that that isn't happening often enough, in large part because too many major companies are publicly traded. But you're proposing something that would make it effectively impossible to happen at all.