There is a quite simple fix that already applies to the IRAs that most people use as their main tax deferral - if you take a loan using your IRA as collateral,

There is a quite simple fix that already applies to the IRAs that most people use as their main tax deferral - if you take a loan using your IRA as collateral, that loan is considered a distribution from the IRA, and is thus taxed. Requiring capital gains to be realized when an asset is used as collateral wouldn't be nearly as problematic as you're making out. For example, if someone's company appreciates to $50M and they then wish to turn some of that abstract value into concrete cash, then yes it's time to pay some taxes. Those taxes can simply be paid with some of the money from the loan too, you know.