This argument always comes up, and it's such a bad one when actually considered. The US was the largest world power, right? They also influenced all other western and/or capitalist nations, either through cooperation or threat of cutting them off from the market.

OK, so consider which communist/socialist countries failed. Did they fail in a vacuum? No. They failed with the "democratic" (often less democratic than the opposition, but this was their messaging) nations opposing them. For example, in Guatemala they elected a leftist president who introduced a minimum wage, increased democratic participation in elections, and the next president introduced land reforms to redistribute land to give it to the peasants.

Guatemala was being used by the United Fruit Company (Chiquita now) for their banana empire. These changes hurt their ability to exploit the people and create more profits, so they lobbied the US to overthrow their government. This led to the formation of a dictatorship and reversal of labor reforms. The dictatorship went on to genocide the native populace.

Communism "always fails" because the most powerful nations in the world are afraid of it, so they do everything in their power to ensure it fails. The few that have been able to survive are dictatorships, because they can better withstand pressure from the outside. (This isn't to say dictatorships are good, just that they're more stable when attacked.) They also generally had or enforced cultural hegemony, for the same reason. It's survivorship bias.

We have no idea how well a leftist government would do if not attacked, because it's always attacked out of fear. If it's destined to fail, why are they afraid of it succeeding?

I'll break this down sentence by sentence.

-According to all data we have, the living standard in developed countries is higher than in developing countries. Additionally, Capitalism is less democratic than Socialism, as Capitalism is a market of competing mini-dictators, rather than democratically run industry.

-Social Democracies a la the Nordic Countries ruthlessly exploit the third world and see rising disparity as Capitalists erode social safety nets. Mass Unionization slows this process, but Social Democracies serve as a good example of why living standards are higher when Workers have more control, and why Unionization alone isn't enough, Capitalism itself remains the problem.

-The USSR is not the only form of Socialism, and additionally it collapsed after it liberalized. It wasn't a Communist state either, as it never achieved Communism, ie it was not a Stateless, Classless, Moneyless society.

All in all, very flawed takes on your part. Attributing quality of life to Capitalism, when it was due to development, is false. Additionally, you falsely assume development only comes from Capitalism, which relies on the absurd logic that believes humans are incapable of developing democratically, and goes against this very website itself.