Maybe that's the part I'm confused about. Aren't they talking about taxing unrealized capital gains, i.e. money that is being left locked up in a single stock, in order to avoid being taxed when it is moved? Wouldn't one of the effects of this tax be to incentive these effective/successful billionaires into moving their money into new investments, instead of being left parked in their already successful investment?

And most entrepreneurs are not, nor will never be, billionaires who are subject to this extra tax. That being the case I wouldn't think that this will put a dent in overall entrepreneurship. It might have the opposite effect, where there are numerically more entrepreneurs making capital allocation decisions, which might be better for society than having fewer people (billionaires) controlling an ever increasing amount of capital.

I grant that tax changes like this have numerous conflicting effects, sometimes unpredictable. Perhaps their will be another art bubble...

I don't think that it's a good thing that we structure our society for the convenience of those who can amass money. I'm definitely not fond of kludgy taxes like this one, but I also don't understand why so many jump to the defense of so few billionaires.

Exactly this. And even worse, you are disincentivizing the most effective leaders from driving their companies to huge success.

It eliminates the risk to established entities to be disrupted.