I think the rationale isn't "do they have more than enough?" it's "is this amount of wealth commensurate with any value provided?"
I imagine this would be a controversial subject anywhere, let alone HN, but I think implicitly is some belief that some individual's (or institution's) contributions are overcompensated relative to their actual contributions. I think as the amount of wealth increases, the probability that that wealth represents some equitable and rational reflection of actual contribution of value relative to others goes down.
The problem is that in any market there's all sorts of "leakage" and hidden unrecognized contributions just as much as there is hidden costs (environmental costs etc) of any business or individual. Also fraud, misrepresentation, irrational decision making on the part of consumers etc.
So my objection to say, Bezos' wealth isn't so much that he has more than he needs, it's that I do not see any way to rationalize his wealth relative to his contributions. The ideas behind Amazon would have probably proliferated anyway, the company is more than Bezos, including the many employees etc, and I think a lot of arguments can be made that Amazon's total societal value is controversial or debatable anyway.
What's a "correct" evaluation of Bezos' wealth in terms of societal value? I have no idea, it's probably a lot, but I do agree that at some point the wealth becomes so extreme that it's more likely to reflect societal and economic failures in value appraisal (implicit or explicit) than some actual justifiable value he has provided.
Regarding (2) we do this all the time in all kinds of indirect ways anyway. There's an assumption there about ownership and the conditions under which that is legally instantiated that is maybe at the heart of what needs to change. If you set up the rules to prevent certain things from happening in the first place, you don't end up "taking" any wealth.
I feel like this is the sort of thing that is so involved it would necessitate a book-length treatment instead of a comment on HN.