It's hard to foresee what the consequences are for introducing a tax on unrealized gains. If the problem is that some people can leverage the unrealized gains

It's hard to foresee what the consequences are for introducing a tax on unrealized gains. If the problem is that some people can leverage the unrealized gains and buy stuff with that, wouldn't the solution be to apply the taxation when the transaction happens? If you're just owning unrealized gains as an abstract number you don't get taxed. If you're using them as collateral for an acquisition or a loan then it can be made the case that it's tax evasion and treated within the legal framework of tax evasion. EDIT: for Musk if would be hard to make the case that he was just "moving investments from Tesla to Twitter" after going public to state his reasons for acquiring Twitter and that he didn't care if it made money or not but he wanted to "fix free speech" or whatnot. That should be a clear case where he's doing extracting value from his unrealized and gains and thus making them realized. You can also make such law apply only to people with over $100M if you want to avoid having millions of tax evasion cases clogging the system