Money is just a means of trading time, time that I put in, for time that you put in.
Some things like being a doctor are harder than being a fast food worker, it takes years of training, and hard work. It makes sense that their time is worth more.
There is a ceiling though where you’re not actively contributing “time” you’ve previously committed time that’s just appreciating because it’s “invested” in paying people for their time. That’s where the problems come in because you have effectively a years work of thousands of people in your pocket, which is a concentration of unchecked power.
Taxing billionaires out of existence ensures that money is invested (in a democracy) by the voters (through their representatives) and keeps the concentration of power from distorting the politics.
This issue isn’t billionaires, it isn’t capitalism, it’s and always has been throughout history, concentration of power. It’s past time we fixed this unforeseen loophole created by the modern world where a handful of individuals become as powerful as a country.
When you have bosses that aren’t “gods among men”, that can’t just buy up their competition to squash it, it’s much easier to negotiate with them to pay you a fair salary. You’re not just a number. Similarly, you can get more done in politics because nobody’s got so much money that they can significantly grease palms/run a campaign by themselves/etc.