It's not like the money is just sitting, liquid in a vault like Scrooge McDuck.
Whether or not you think any of the companies funded by YCombinator[0] are actually worth their valuation, you have to realize that there will be fewer such startups if a tax on unrealized capital gains is passed, and that VC activity, along with the future startups chasing their money, absolutely will move to countries without such a tax.
Again, maybe you actually believe the startup scene in the US is worthless, in which case, go ahead and advocate for an unrealized gains tax Just be honest with yourself that it will entirely shut down sectors that others view as critical to the country's future dominance.
That's a bold claim. The tax-averse amongst us say that, but in my experience investment flows to the best ideas / best distribution / best businesses. If those people are in the US because they're citizens, the capital will flow to the US, and investors will take the hit.