Basically, if, say, Jeff Bezos never existed, it seems likely that someone else would have created Amazon, perhaps a few years later, and maybe not quite as good. "Big online retailer" is sort of a natural niche, with a bit of a natural monopoly. So while the classic argument for letting people keep most of their wealth gained in the free market is that they provided a lot of value, maybe that doesn't make as much sense when you consider that if they hadn't done it, somebody else probably would have soon after.
It's obviously impossible to gauge exactly how much excess value someone provided compared to the world in which they never existed, but to round it up to 90% or doesn't seem very accurate.
So it seems like in our current world, there's a winner-take-all effect that a lot of venture-backed startups are trying to exploit. If we were a lot more aggressive in taxing companies like Amazon, my intuition is that it would go a long way toward reducing this effect.
(1) https://astralcodexten.substack.com/p/billionaires-surplus-a...