Well we're about to, as it's poor people voting for this.
Very few so-called "billionaires" have billions of dollars in cash. Most of it is tied up in non-liquid ownership of their companies or their real-estate. Let's take on billionaire in California that can't "flee" like the tech billionaires, Donald Bren. Bren owns the Irvine Company completely; it's a private entity that owns around 125 million square feet of apartments, retail centers, and office buildings. Irvine Company is private, but his estimated net worth is $19.2B because of his ownership in the company and the real estate assets that underpin it; it's not cash.
There are two major problems with this. The first is "estimated" net worth; what is that? What is the value of his holdings? One could argue the value of the real-estate, but they fluctuate all the time due to supply and demand. If there was a sudden increase of real estate on the market, that would drop the value; what's it worth now?
And why would there be a sudden increase of property on the market? Simple, because of the second problem. If we assume his net worth is $19.2B, then he would need to pay $960M. He doesn't have that cash on hand, so he has to do one of two things: sell property in his portfolio (many of the targeted people for this tax are real estate developers/owners), increasing the supply of real estate on the market and depressing prices all around for everyone, or increase his rents to his tenants, who are all businesses who employ people. If the rent goes up, expenses go up in all businesses, meaning they will then lay people off or pass on the higher expenses in prices to consumers of their products. Most likely they will do both.
And absolutely believe these billionaires, who can directly influence companies, prices, and the like, will make any tax pain they feel punitive to all California residents, and for one simple reason: the tax is only one time, until the next time. If it can get voted through once, it'll be voted through again because the ballot initiative is clear, it's a "one-time tax paid over 5 years" to pay for "healthcare and public services" which are by nature on-going expenses; when the 5 years are up those expenses will still be there, requiring another 5% asset tax.
So you'll see companies lay people off, you'll see services go down, you'll see companies close, because the billionaires will have to sell assets, which are always companies that employ people, just to pay the tax. Hence, because of poor people, poor people are going to be laid off.