I do think governments should explore taxing unrealized capital gains too, though.

Oh, boy do they ever.

One big issue is that they can take out loans with stock as collateral. Yes, they eventually have to pay the loan back (and sell stock to do so, thus paying some capital gains), but they can still get around paying their fair share of captial gains (eg: sell the underperforming stocks to minimize capital gains, or just take out a new loan to repay the old one). While I can see the benefit of being able to use your stock as collateral for a loan, there needs to be changes to how capital gains are calculated in this case.

Another issue is that when they die, the inheritor of their stocks gets them with the cost basis reset (stepped-up basis). Let's say I have stock that I purchased for $10/share. I die when the price is $100 and leave it to my sister, and she sells it when the price is $110. She only has to pay capital gains on $10/share, not $100/share. If you combine this with a cycle of taking out a loan to repay your previous loan until you die, this means that your estate can settle the final outstanding loan with virtually no capital gains tax at all, since the stepped-up basis for your stock goes into effect once it goes to probate (ie: before being distributed to creditors and beneficiaries).