According to Wikipedia/Forbes [1], the number of billionaires in USD is 2,781 globally so they know this proposal has no negative political impact (and obviously it is easy to say that the public overwhelmingly supports this since it essentially concerns no-one) but it is really only posturing, with a dose of ideology and demagogy (it's to fight poverty! Many other quotes from the article are also rather scary), because this is neither needed nor effective for the stated aims. This would most likely also only benefit already rich countries, not the poorest ones, because that's where billionaires live.
Practically speaking, taking 2% of the company stock per year would expose the govt to downswings, and if they own enough of it, potentially get them involved in things like board seats. Public politics and corporate politics probably shouldn't mix. But excise taxes exist for other kinds of assets, and people find the cash for them. Selling stock to someone and forking over the cash from the sale (beware double taxation on the sale) doesn't diminish the overall value of the company, it just spreads ownership around. But it's sort of a weird concept.
