First of all, if a person or company were sitting on $100 trillion in cash it benefits the hoarder of this cash because they now wield inexorable control over the economy. In a nation like the US this is counter-democratic.
> You could make the argument that the cash has the potential to be spent in large influential harmful ways, like on elections or something, but that's a very different topic.
It's totally not a different topic. You can't talk about the dimensions of money like they aren't interrelated. A wielder of $100T has the potential to harm. Whether it's because there's already too much liquidity in the economy and movement of that $100T would cause massive inflation, or because there's too little money to make the economic engine hum the $100T would be much better off in the hands of those who would send it flowing through the veins of the system. There's other less straightforward ways it could work -- the economy could start tilting to service the needs of the entity holding $100T (e.g build $1T rocketships to mars rather than planting food for everyone else). In general lack of economic diversity is going to cause issues.
> You can deduce that cash on hand harms nobody by doing a thought experiment.
If you have $100T it's going to grow or fall, and the process of getting there meant taking liquidity out of the system. Liquidity is super important, after all, economic demand is not "I want X," it's "I want X and am willing and able to pay for X." Can't vote with dollars if you haven't got dollars. The latter part of this is what fails as cash pools into certain economic outlets (read: billionaires).
The fed, appointed by our elected government, uses monetary policy to try to avoid this outcome, but if a small group of unaccountable private individuals who have a lot of money end up wielding this same power, it gets bad -- the effect that billionaires have is that they attenuate signal from the economy about what goods/services would most benefit humans in aggregate. The efficiency of capitalism dies when all the decisionmaking is left to a few parties. The beauty of capitalism is most apparent when there is a diversity of demand. This is why many are calling for fiscal policy with the goal of rebalancing the "dollar vote."
At a minimum cash that isn't moving isn't being useful in the economic system, and that can be hurting folks who need that money to be put in their hands to drive demand. I don't think you can say today's system is in any way near perfect, since folks still die for want of food and medicine because cash has not been mustered to direct the economy in a direction that spreads the wealth.
The new hotness is the idea that if you put money in the hands of the people (e.g UBI but also smaller ways like making transportation, mail, schools, healthcare more affordable) you steer the economy in a direction that produces goods and services that lift everyone's quality of life, not just the quality of life for a few. The rich may pay, but they get the money back because they have capital assets and the economy doing well increases those assets. A wealth tax is one way to help us head in that direction -- the direction of rebalancing economic control to make things more sustainable and good for most humans.
Oversimplifying of course because this is in response to a thought experiment. Sadly we are also failing at dealing with capitalist externalities like climate change and decimation of our natural resources.