I think we basically agree on this. The question is whether a flat policy of "nobody can ever control more than a billion dollars" has any justification, either pragmatically or ethically. "Wow that's a lot" and "Maybe they aren't actually very smart" aren't really that well founded.
This is how you end up the management class of people who do basically nothing and collect huge bonuses for it, running companies into the ground slowly by not making them taking any risks/not trying to do anything.
Often that's exactly what prevents bad ideas and gives a chance for good ideas to flourish. The actual strength of democracy is not that it allows to make more decisions, but that it prevents making decisions unless they seem good to a lot of "bosses". At least that's how it works in multi-party democracies. Systems of governance that concentrate a lot of power in single hand may have a good run from time to time when the supreme leader accidentally has a good idea and can move fast with it, but they fail miserably over the long term because they have no safeguard against bad decisions which are way more frequent.
There's nothing unique about capital and its derived power that suddenly makes concentrating it in one hand any less harmful then it is with armies, countries and nations.
Doing nothing is the best course of action more often than any other sort of action. Economy loves stability so it can stably grow. Some amount of disruption is necessary for the development but it's really hard to tell if the billionaires capital reinforces or suppresses the right kind of disruption.
It's still the same primate but now it's got a gun.