Using the market value as collateral is in fact one way of realizing the gains: the investor is using the loan to convert their gains on invested capital into something usable. The capital gains tax would only be triggered when the investor utilizes a price other than their basis cost for their financial instruments.
This would probably not affect very many people: 99% of people don't use their retirement stocks as collateral on loans. It would fix the "Jeff Bezos et al. never pay taxes because they just keep getting bigger and bigger loans to pay off their loans" nonsense.
I don't think anyone in D.C. is currently proposing this, but I think it's a nifty idea. Even if the tax revenue generated is modest, it would boost the average citizens confidence that the system is working and not rigged/broken. And that is probably something worth pursuing these days given how dissatisfied voters have been for the past 9 years or so.
This might be the larger problem with this, since we probably, culturally at least, want more family owned farms and less corporate monster farms. This would not help the current trend away from family owned farms.
That said, it's an interesting proposition.
You’re talking about behavior which is only taking place as a part of the tax dodge: taking out loans with stock as collateral.
Worst case scenario is the ultra-rich sell their stock bit by bit.
Or they donate it to charitable foundations they themselves manage.
Actually weird of so few people make use of it.
There are small edge cases for "the thing I own is worth a gazillion dollars now but I never want to sell it." Those edge cases already exist with the "I grew up in this house and I am emotionally attached to it" situation. It sure seems to me like people having unrealized gains in equities is, you know, vastly more common than finding out that the weird knick-knack that reminds you of your mom is actually a valuable collectable worth millions.
Valuable possessions have a tendency to move to areas where ownership is not taxed.
Thus only unmovable assets are taxed simply for possession.
That’s what California’s Prop 13 was supposed to address.