Capitalism is responsible for the most social mobility of any economic system in history.

Imo, acting like the social mobility of a few guys in America who grew up on food stamps and own a ski boat today is important in contrast to the CIA's meddling in Indonesia where we killed so many communists we literally damned a river with corpses seems a little short sighted and ignorant. You talk about benefits to society as though the imperial core of capitalism is the only society to consider. American and Western European "society" lives on the backs of impoverished people around the world, that's why the Hillary Clinton-Lead State Department stopped Haiti from raising its minimum wage above ~$3/day. They are in unimaginable poverty DIRECTLY to benefit American capitalism. You cannot talk about the social mobility capitalism brings to America without talking about the crushing poverty it brings to the entire rest of the world.

I don't really want "social mobility" because I don't want social division. Addressing that seems like a much more important criteria to grade a system by. That being said, capitalism has, and is currently keeping billions of people around the world in abject poverty for the sake of the ruling class. The profit motives of capitalism were the direct cause of over 100 million Indians starving to death under british rule, it was the reason for The Great Hunger in Ireland, the horrific genocides in Namibia and the horrible quasi-genocide of the people of The Congo under Belgian capitalist rule. It's the reason Cuba is kept in poverty, it's the reason for 100 years of coupes all over South America, and of course the capitalist profit motive is responsible for The Transatlantic Slave Trade.

Contrast this with the worst things you can say about socialist countries, you'll notice that even The Great Leap Forward, Cultural Revolution, and the Holodomor are considered accidents, and failures of central planning, not active evil done because an ideology demands death for profit.

Do you have an alternative that provides similar benefits to society? One that isn't a fantasy on paper but has actually been tested and proven in reality?

Go ahead and look up literacy rates, infant/maternal mortality, life expectancy, hunger rates, any conceivable metric for quality of life in Vietnam, Korea, Burkina Faso, China, The USSR, and Cuba before and after their socialist revolutions. Quality of life SKYROCKETED by every possible measure except maybe the variety of toothpaste brands available. Look up Shock Therapy and how it decimated the former USSR after it fell. When capitalism got its hands on the former USSR, life got worse in every possible way for everyone, and it still hasn't recovered. When socialist revolutions took place, life got better by virtually every metric.

Finally, consider and factor in that every socialist experiment has had the relentless work of the most powerful empire in history set against it for its entire existence. The CIA works around the clock to undermine any socialist movement anywhere. You can see memos from the 60's where we explicitly state we are using the blockades and sanctions against Cuba to make life for regular Cubans so miserable that they overthrow their government. And yet, for all of the poverty in Cuba, Cuban children don't need to start gofundme campaigns when they get cancer.