In software engineering divide and conquer is considered a key pattern. In business, it is widely considered that large monolithic entities are the most efficient at delivering results.

My point of view is that these large entities seem and have the power to pretend that they are efficient. In the long run, as with all monolithic structures (think large cities, geographically large countries, large monolithic software projects), the inefficiencies start tearing them at the seams, each in different ways.

Ultimately, if large megastructures were the most efficient, then humanity should strive for a single country (or a corporation) to run the whole humanity but that's never the case - never was, never will be.

And that's tightly related to your argument that billionaires should not exist. As another commenter pointed out, it's difficult to calculate the wealth of a person. Therefore, we should make it easy. Is your cash and your assets worth over X amount? If so, choose which ones to transfer to public national escrow during the upcoming fiscal year. It's not perfect, and I am sure clever lawmakers could come up with more nuanced and fair approaches, but it's better than the current state.