Capitalism so far, hasn't worked without socialism. Every few years due to the boom and bust cycle, a.k.a. "the business cycle", capitalism has to be bailed out with public funds. That's essentially socialism saving capitalism every few years.
More, why does socialism have to completely replace capitalism overnight in order for socialism to be a legitimate or viable candidate for eventually replacing capitalism? How many centuries did it take capitalism to replace chattel slavery and feudalism? It took the mercantile class and its republicanism, centuries, to replace its predecessors. The capitalists didn't show up on the scene one day and with a single swoop of the sword, defeat all of the kings and nobles of Europe, replacing slavery and feudalism in one decisive victory. It was a process that took hundreds of years.
According to Karl Marx and Frederick Engels, a mode of production or socioeconomic order, doesn't replace another, until the material conditions are ripe or ready for that new economic system. It took a revolutionary capitalist, industrial movement with all of its new technology to turn the merchants and the powerful industrialists of the 19th century. Republicanism the child of capitalism, didn't replace the monarchies of Europe, until the industrial revolution was well established. Capitalism in Western Europe, became the successor of chattel slavery and feudalism. Constitutional, parliamentary republicanism, funded by capitalists, replaced the power of the kings and feudal lords,
That process took centuries, not one day or a few months, or years. Marxist socialism was born from the industrial revolution and it will take some time to fully replace capitalism. It's not going to happen overnight. There are no single decisive victories, but rather a string of victories and losses, failures and successes. It's a process of democratizing and socializing production (our economy), with every new technological and social innovation that contributes constructively to that transformation.
In the 21st century, we have advanced automation technology, in the form of autonomous machines, like robots and self-driving vehicles. We also have AI, i.e. Artificial Intelligence, which makes machines intelligent and capable of producing goods and services without much human involvement or assistance. Before the advent of computers, and the aforementioned advanced automation technology, machines had to be animated, operated by human beings. Production followed the rhythm of the human organism, of humanity, rather than the algorithms of advanced computers, robots and artificial intelligence.
The 21st century seems to be the one that will reveal socialism as the successor of capitalism, forcing society to adopt it as its primary mode and system of production. Intelligent, autonomous machines/robots/automative systems, don't rest, eat or need all of the resources that a human worker does. They function 24/7, producing all of the goods and services that we consume.
