There is no intrinsic reason why socialism could not one day "prevail," in the sense of becoming ascendant in some country. It's perhaps less likely that socialism could become the dominant economic system in the world, if only because so many countries now are not socialist  (and are instead mixed economies) and it is possible that should major shifts occur in the global economic order countries may shift in highly varying ways. Still, when one considers that 500 years ago, much of Europe was dominated by feudalism, while societies in the Americas ranged from city states (for example, the Mayans) to indirectly ruled empires (like the Aztecs), great upheavals certainly seem possible.

Many countries have had governments that were communist or Marxist-Leninist, though some of these countries had multiparty systems in which the communist party won an election. Only five countries -- China, North Korea, Cuba, Vietnam and Laos -- are communist and one-party states, though all of these countries except North Korea have introduced varying elements of a mixed economy. Still, one-party communist countries often call themselves socialist states. They also tend to limit personal political freedom (in addition to economic freedom). The obvious undesirability of this condition for a majority of citizens makes it less likely that a country's populace would support a shift to communism, meaning that future communist governments may have to be imposed upon a people.

The list of countries that have had socialist governments is long. The list is even longer if you include coalition governments that included a socialist party as a partner. But the term socialist, even when applied in the context of a democratic republic (e.g., Costa Rica) or a parliamentary democracy (e.g.,  Estonia), can mean vastly different things. Socialist parties around the world vary widely, and they often differentiate themselves by aligning with like-minded parties in different international socialist groups, like the Socialist International or the Party of European Socialists, which often represent philosophies that now resembles democratic socialism -- socialism achieved through democratic means -- rather than "pure" socialism driven by authoritarianism.

One of the problems with socialism is that instead of focusing on self-interest as the motivator, it relies on peoples' interest in caring for one another -- a much harder sell. So as opposed to capitalism, which approaches the issue from a hands-off perspective, socialism requires a person to care for other people. Lots of other people, and often people someone has never and will never meet, some of whom truly need assistance, some of whom don't.

The problem is, that's a serious strain on the working class and the amount of money they can scramble to earn, so some countries that leapt on the bandwagon and started programs like public housing and Social Security have started to reexamine and realize they're not sustainable.