Is this really... true? I get that there's this idea in people's heads that you couldn't tax for example some incredible rare art collection, because who is to say it's true value and you damage the owner by applying such a tax burden they actually have to sell the art. But let's take a step back. Where is the real money? Bezos, Gates, Buffett, Page, Brin, Ellison, Ballmer, Bloomberg, Zuckerberg. Further down the list the same thing is true - the people with wealth actually have the vast majority of their wealths in companies. It's not difficul to liquidate, it's not difficult to value them, it's actually quite comically easy. Even the guys who actually do have enormous art collections are almost all art dealers, they can and do liquidate their investments all the time.

On to the core question: how much is the stock in the non-public company you work at really worth? I don't know, sell it and we'll find out. A SWE with illiquid stock probably actually has a much shorter timeframe of when they want a liquidity event than the US government has.

The truth is that the reason it's difficult to tax rich people is because the first thing anyone does in this conversation is talk about how hard it would be, without thinking for a second if it actually would be hard. Let's lay aside any talk of real wealth taxes, and just stick to our current tax system, Elon Musk borrowing against his Tesla stock is not a taxable event. Let's change that today. It's specific, easy, would immediately drive revenue, and doesn't require any innovation around true wealth taxes. But... we won't, because it's not difficult, the government just doesn't want to do it. What does that mean? The burden of tax is primarily put on income not wealth, meaning your SWE with non-public stock is already getting absolutely pounded with taxes on their income instead.