It doesn't necessarily discourage investment much from the perspective of society as a whole - 1 person worth $20 million isn't obviously making way more invest

It doesn't necessarily discourage investment much from the perspective of society as a whole - 1 person worth $20 million isn't obviously making way more investments than 2 people worth $10 million (or 4 people worth $5 million). It's also not obviously an improvement to morality for a society to worry about someone worth $10 million continuing to increase their net worth while looking the other way while children (who in absolute numbers surely outnumber those worth $10M+) lack food, housing, basic healthcare, etc. Just because we are currently making one set of choices does not mean we could not be making a different set of choices.