They didn't used to have billionaires, when Mao was in power. Strangely, a lot of Westerners seem to hate what China is now, but also hate what China was before, which seems very strange to me. "The reforms were bad and China should be more Maoist" that is a coherent position, "The reforms were good but should have gone further and become even more capitalist" that is also a coherent position. But "China is bad now because they allow billionaires but they were also bad when they didn't allow billionaires" isn't really coherent, it just seems like knee-jerk opposition to whatever they do or don't do. Anyone who wants to offer critiques should be able to answer the simple question of whether the reforms were good or bad, whether they moved in the right or wrong direction.

On the off-chance that you actually want a serious answer to the question, let's start with some history. Before the revolution, China's average life expectancy was in the 30's, it experienced frequent famines, had no access to medicine, and had had similarly awful conditions for literally thousands of years with no improvement for the average person. The revolution brought about huge improvements in life expectancy and the basic necessities of life (with the failure of the Great Leap Forward being an exception to this general trend).

However, while central planning works for providing basic necessities because the demand for those necessities is very predictable, it is not the most efficient regarding consumer goods. This was one of the failures that caused the collapse of the USSR, the desire for consumer good and the inability to compete domestically. Furthermore, despite the increases in life expectancy, virtually everyone in China still lived in extreme poverty. The reforms in the 80s were intended to address this, and they were wildly successful at it:

In the words of Deng, it doesn't matter if the cat is white or black, so long as it catches mice. It doesn't really matter if a policy is ideologically "pure," so long as it provides clear material benefits for the common people.

Where I'm standing, the whole point of having any sort of political beliefs at all is to figure out which policies will improve people's lives. And lifting 800 million people out of extreme poverty over the span of 40 years, can you name a more successful policy, ever, in all of history?