Fair question, honestly I think it’s ‘hacky’ and there are cleaner approaches. Regulating unrealized gains stands to cause a headache and costs for those navigating an already very complicated tax system.

To work the implementation would require everyone’s assets to be monitored and reported - extra work for tax software, brokers and taxpayers. Plus the development costs to comply with these regulations will be passed onto taxpayers and investors. Yes additional taxes would be recovered but are a drop in the bucket on the scale of the US economy.

To a lesser extent it also discourages regular folk from financial planning since it creates a public perception of “why should I own stock if it’s all going to be taxed anyway?” further concentrating influence on institutional and super wealthy investors.

While I’m still undecided on it, the solution others have proposed of regulating borrowing against collateral seems fairer as it puts the onus on the borrower to carry the admin overhead - regular investors remain unaffected.

Now my hot take - I think this is all a distraction from the real problem with the tax system Step up in Basis. If it wasn’t for step up in basis the gains would at least be taxed on the investor’s death anyway, but right now those who inherit stock get to permanently avoid the taxes on the gains that occurred during the original owner’s lifetime (up to a certain limit). I suspect the politicians responsible for passing such legislation would be too directly affected to address that one.