My argument is that if you take a billionaire and take away 90% of their wealth, they're still filthy rich. If the way you do this is by taking away 90% of everyone's wealth, you'll make everyone except the rich suffer badly, and when the dust settles, the rich will still be ahead. Allowing for large-scale failures like these only creates lots of unnecessary suffering and burns a lot of money.

As for whether "the rich gaining large sums of money doesn’t matter much either" - if you're asking if it doesn't matter much either to them then yes, this is the flip side of the marginal value of money point. A person with $1000 in the bank will be much happier about getting extra $100 than a person with $1M. It's one of the core (if not the core) argument towards reducing income inequality - moving some money from the rich to the poor is a net increase in happiness and value in society.

One strong caveat being, building things that require big capital expenses, like factories or specialized machinery or infrastructure. These things require being able to front up a lot of money, and at the same time, they don't mesh well with democratic decision-making. So you want to have some way in a society for an individual, or a small group of individuals, to be able to take control over large amounts of money - otherwise factories and bridges and utilities just won't happen. This doesn't necessarily mean you have to allow that small group of people to capture the entire wealth they generated, though.