In theory, there is nothing wrong with billionaires existing.
In practice, the presence of a class of ultra-rich people almost always leads to a tightly networked society that can disproportionately influence policy to their benefit and at everyone else's expense. They can influence elections and any media they own, they can have the ear of politicians through lobbying and exclusive dinners and gifts, and they can coordinate much better than the commoners because there are fewer of them, they have shared interests, they attend the same clubs, their kids attend the same private schools, and so on.
Yes, in theory, a country could childproof its laws enough to prevent an immediate plutocratic coup. In practice, the plutocrats will never relent in their efforts to bend policy and laws to their own ends, and safeguarding laws from them requires a ceaseless watch and united front that general publics usually can't manage.