Fortunately, capitalism is a fairly young system & requires a government to provide violence (police & military) to preserve it. Capitalists often ask government to step in and make capitalism viable long-term. That is, keep capitalists from destroying each other. And to keep the populace from trying alternatives. ("Revolution.")
Slavery was more stable: lasted millenia. We're in turbulent times, particularly since we're probably ending neoliberal capitalism: an unstable form of capitalism whose one advantage is to damage the imagination enough to stop seeing alternatives except a single bad one.
I would be surprised if the death knell for neoliberal capitalism is really sounding. And what, pray tell, is the replacement? Fascism? Plutocracy? Probably not Socialism or Communism...
And slavery required less violence, per capita, to maintain? I don't think so. Innumerable slave revolts. Wars of conquest to capture slaves. Slave raiding. The daily violence needed to enforce slavery. Sexual violence perpetrated on slaves.
Capitalism is a rock of stability, from a civil society perspective, in contrast. When was the last time a developed capitalist nation had a civil war, or even a major violent revolt?
Slavery was more stable: lasted millenia. We're in turbulent times, particularly since we're probably ending neoliberal capitalism: an unstable form of capitalism whose one advantage is to damage the imagination enough to stop seeing alternatives except a single bad one.