More like a symptom of a broken broad economic system. In all forms of capitalism, it is a given that much wealth accumulates in the few. It's a system where resources are distributed based on capital, and capital is a resource, and it's a system where those with more capital have more voting power both economy-wise and politics-wise. There is no such thing as a capitalist economy that has even wealth distribution long-term, it was quite plainly a system created for the sole purpose of keeping those with power in power – this isn't an exaggeration, the guys who basically created/popularized modern capitalism and are the basis for all the writings and philosophy of the "founders of capitalism" were post-french revolution aristocrats who wished to push a system where they could keep their power instead of having it taken while also not having their heads chopped off.
Even with the best taxation capitalism can offer, there is no solution to the capitalist problem. It's a system that requires there to be suffering underclasses and carefree upperclasses. It requires an immoral social hierarchy to exist. The systems that reduce the damage of this innately bad hierarchy while still maintaining it (welfare corporatism, for example) are incredibly unstable over the long-term and inevitably result in a populace that want to tear it down. The people who receive the most benefits from welfare & social safety in a capitalist society are often the ones that are the quickest to tear it down (them, and the elite) and guide us back to right-wing feudalism.
Billionaires might maybe go away if we "properly" tax, but there is only so much you can do to patch up a fundamentally broken system. The countries with the most wealth equality and highest wealth taxes also happen to be countries with a ton of megacorporations and/or billionaires... Switzerland, Scandinavian countries, Finland, Germany, Australia all have the highest wealth equality while all being on the top 15 for billionaires per capita excluding extremely small nations. Plus those countries have a tendency for alt-right movements to pop up, a few even more by proportion than the US...
TL;DR capitalism bad socialism good eat the rich