Edit: typo.
Present day accumulation of wealth creates a huge power imbalance, billionaires are citizens, just as I am, why should they have more power, as a citizen, than I do?
If you defend that billionaires should exist then, in my opinion, it's required to design a way to protect the rest of society from this class of people amassing more and more power.
The problem isn't money, it is our power structures. Right now, being a billionaire is almost inherently bad, every billionaire is a policy failure.
Besides that, hierarchies of power exist outside of realm of money. And they are a natural occurrence. You will not only find billionaires with more power than you, but also a lot of other citizens that belong to these other hierarchies that have achieved their power via their competence.
Power in itself is not dangerous or undesirable, power that becomes tyrannic is.
What is more dangerous is inequality, specially inequality of opportunities. Certainly some times of inequalities of outcome can also be dangerous in certain situations if not kept at check, basically because they may become a source of violence.
The accumulation of wealth by a single individual is not inherently harmful for society, it all comes down to the intention for which accumulation happens and how is that spent.