Arguing that some people demand aspirational goals as high as that in order to do what they do begs many questions: Not that their demand is invalid (as demands go, I've never found bargaining very easy, arguing for billions is beyond my comprehension) but I seriously doubt the ability to add that much value is a skill distinct enough that it can command that rate of return, against all other choices.

Not "they can't generate the ROI" as much as "others can generate that ROI just as well, it's contestable".

Fund managers (for example) vary in costs, and returns, in ways which I think undermine the proposition here.

Many people take remarkably "fundamentalist" views of peoples rights to limitless potential. "it's my right to accumulate billions if I want" is very strong in opposition to this, aside from the people who may plausibly earn billions who have an entirely rational reason to disclaim alignment here. We experienced this 6 orders of magnitude down the tree in an australian election around tax concepts called "franking credits" which were proposed to be ended by the labor party. Large number of pensioners voted against it, despite being repeatedly told they had none, and would lose none: The meme of "Labor is coming for your franking credits" was unstoppable.

Many people who will fail to become millionaires, will oppose constraint on becoming Billionaires.

A parallel but unrelated case: Tax on lottery winnings. Fair or unfair?

It’s the value of the shares that make them billionaires.

Is the politician suggesting they should liquidate their shares? Who will own the company then? Will they keep changing ownership and diluting shares if it reaches billion dollars?

This argument doesn’t even make sense.

As far as taxes go, these are unrealised gains. Even for Capital gains to trigger, there has to be some kind of transfer/sale. And transfer/sale has a different meaning in tax and legal language.