From what seems to be the intention of the tax plan it would be regardless of if the asset was available in a stock exchange so if the value of Tim's shares put him above the threshold he would still have to pay the tax. His wealth through epic would likely be less then if it were a publicly traded company though. The effect of paying tax on shares was always true as it becomes a realized capital gain and is already covered under the current tax system.

He could also attempt to sabotage the company's valuation in an attempt to maintain control.

As long as he has no legal fiduciary duty, it seems like an obvious thing to do. It's definitely what I would do.

And in 4 more years the new tax laws could be anything.

The stories I see, like https://www.cnbc.com/2021/10/25/senate-democrats-push-for-bi..., says the tax on unrealized gains only applies to "tradeable assets." It doesn't seem like that would cover private companies. From what that article says, it would invoke an increased tax due to the deferral of taxes though.

there are liquid secondary markets.

Yeah, maybe they would count that.

Are those markets liquid enough to support large sales by the owners though?

It still seems problematic. Even if my company goes public, if I haven't actually sold the shares and made any money, why should this be taxed? If there are loopholes that let the founder avoid those taxes at sale time, fix those, but otherwise why should we effectively take away ownership of a company just because someone has built it up successfully.

One loophole is the step-up basis that applies when the owner dies. Not sure why that wasn't targeted instead (maybe because the current plan produces revenue more quickly?).

"Billionaire tax" plays better than "death tax" so we get tax policies that fit into 30 second commercials, not those that make economic sense.

So why not a "dead billionaire tax"? The heirs to fortunes seem likely to get even less sympathy than the ones who actually built them.

Reads like a cash grab to me.