Besides the obvious, the hidden benefit of this is that it provides an endpoint to runaway growth.
The biggest problem with capitalism, the reason it's so destructive to the planet and to the workers and even, ultimately, to the capitalists, is that, after a certain point, the money's just a way of keeping score. The number at the bottom of the column has no bearing on what you can buy or do; as a result, there's no such thing as enough. The number can always be bigger.
Under this proposal, once you hit $1 billion, you've won capitalism. You beat the game, achieved the maximum score; you're finished. There's nothing more you can accumulate. You now have to find a purpose in life other that the relentless pursuit of profit. (And if we're really lucky, it might be something that actually benefits other people, but even if not, it's unlikely to be as damaging as whatever it is you were doing to get that $1 billion.)
Instead of companies expanding endlessly, like tumors, there's a point where, when all the major stakeholders are maxing out on profit, it makes sense to just hold steady. Keep doing/making/selling whatever it is you do/sell/make, but stop trying to do/sell/make more of it every year.
The problem with a tumor--what makes it cancer--is that it keeps growing and growing, until eventually it's taking up so much space and consuming so many resources that the surrounding tissues can't function. The tumor doesn't have to do anything better than the other tissues in order to crowd them out; it just does it faster. Stop the uncontrolled growth, and it's something you can live with.
Stopping the uncontrolled growth of capital means more opportunities for multiple businesses--big and small--operating in the same sector, since it doesn't make sense for any one company to gobble up too much of the market share. That, in turn, means more choices for customers--and workers, since they can take their skills to another employer doing similar things. It means less waste, as there's no longer an economic upside to spewing cheap goods out of a fire-hose before you even know whether anyone wants to buy them. That could mean slower, more thoughtful use of resources in the first place, but at minimum, it's going to mean not manufacturing products only to immediately throw them away.
