Capital has to compound exponentially on aggregate, because if there is no return on capital, there is no reason to make new capital, and production grinds to a halt. So if it has to grow on aggregate, the fact it sometimes go down merely increases concentration of wealth, as it's then at a discount for those who have greater means.

Capital has power over society because society is reliant on material production. Being able to grant someone enough money that they no longer ever need to work (or merely enough for them to have a decent livelihood) is significant power over them and motivates them to do what you want. You can then use this kind of power over the media, politicians, etc..., and you can struggle to have the rules be to your benefit.