Around the turn of the last century the answer seemed clear. Instead of being owned privately (like a personal item) society’s major productive assets should be owned socially and controlled democratically. During the nineteenth century Marx had laid out the theoretical basis and by 1900 his works had been translated into all the major languages. The question re: the paradigm of social transformation seemed settled.
Then there was a schism. The movement bifurcated around 1920. Communism was revolutionary, Social Democracy was reformist. Each of those currents had millions of followers worldwide during the middle of the twentieth century. There was much revolution and there was much reform.
Those movements had great energy for many decades. There was one attempt after another to implement their ideas. All kinds of variants.
Ultimately, neither communism nor social democracy was able to replace capitalism.
. . . probably is the key geopolitical question of our times.