First, this creates a perverse incentive for founders to stifle the growth (or more specifically, in private companies: valuation) of their companies in order to avoid a high-tax event. This may actually be something the Board would approve, in some companies, because this high-tax event would involve the dilution of founder control; companies grant founders tons of shares for a reason, its because they're assumedly doing something to deserve it, and if the Board wanted those shares in someone else's hands they would have done that in the first place.
Second, this is again especially true for private companies but also applies to public ones: it adversely impacts American companies and American founders. We are a global society. If a strong, American company is forced to sell a significant portion of their shares, either on the public market or through private investors, in order to pay a large tax bill, who is going to buy those shares? Investors, generally, which includes entities both domestic AND FOREIGN. These taxable events will move significant wealth out of American hands and into foreign investment entities who are not subject to the same laws.
Third, many people don't realize that the total valuation of the stock market outweighs the total value of all US Dollars in circulation by ~30x (~$50T vs ~$1.5T). Tim could not be more right when he says that he's not really a billionaire, and Musk is not an almost-trillionaire. Stock wealth ISN'T REAL, and I don't just mean that in nebulous, floaty terms, I mean it in very physical, real, "If Democrats try to make stock market wealth real, they will destroy the economy, full stop, there are literally not enough US Dollars to represent the wealth of the US Stock market in real terms." Because taxes are paid in US Dollars, not in TSLA shares, these taxable events will reify a MASSIVE portion of fake-wealth currently tied up in the stock market, which is already significantly inflated due to the Fed's QE over the past decade.
Fourth, this is basically a "Delaware C-Corp" situation; the only reason this tax bill would generate so much income for the US Government is because it is new; billionaires haven't prepared for it. So, it'll make a ton of money, ONE TIME. Looking into the future, its unsustainable; it creates a massive incentive to offshore personal founder wealth, and possibly even citizenship and corporate headquartering. With COVID accelerating remote work, there's a ridiculously real possibility of companies, in the near near future, being founded outside the US, but still accessing as much US talent as they need. The USG can tax the middle class people who work for the company within their borders, but the rich founders? They're on an island. Ayn Rand was an idiot, but she wasn't wrong about everything; rich people will do anything to escape a tax bill.
But the dumbest thing, above all else, is how fucking self-servingly idiotic this bill is. I know our policymakers all share one brain cell, and Biden has been using it a lot recently, but this is beyond reproach. The democrats need money to pay for social programs. That isn't wealth redistribution; its a handout. That money will come from taxing American businesspeople, forcing them to sell shares, which will then be bought by other corporations and investment entities, and the wealth gap will just keep getting bigger and bigger; on the one side are average Americans who can't breach into the INSANE wealth inside corporate America, but are at least kept going on food stamps and free childcare. On the other side, people still insanely rich, just a little bit less so, and their companies are now owned by Tencent and the CCP.
The better path forward for some kind of wealth redistribution bill is something which creates incentives for companies to distribute significantly more corporate shares to more employees. I don't know what that looks like, specifically, but: Every single employee of every single company should see a portion of their compensation be represented as shares in the company, even if its just a handful, from the Waitresses serving tables at the Cheesecake Factory to the CEO who plans strategy. Ultimately this benefits the company, at least in some way, because it aligns financial success at every level, it benefits employees because its an additional vector for compensation which is anti-inflationary at a macroeconomic level (unlike a minimum wage increase, which is absolutely still necessary, but I don't feel is enough), it benefits tax revenue because many lower-wage roles will liquidate these shares very quickly (if possible) (a taxable event), and it benefits US political policy because it keeps ownership of many American corporations in the hands of Americans, rather than foreign investment entities.