how do you explain that trade and prosperity have, at every point in history, gone hand in hand?
Trade increases prosperity, but it also increases inequality. There is plenty of prosperity in America, but is mostly going to highly skilled people and people that own capital.
So what is the solution? Many people, including Donald Trump and Bernie Sanders, think the answer is protectionism. But that has been tried, mostly unsuccessfully, many times before, and falls into the category of "simple, obvious, and wrong". Liberals tend to say the solution is "education", partly because they see that as the solution to everything, but government promoted education schemes don't have a good track record of getting people into jobs. Taxes on the rich to fund handouts for the poor, kills initiative, and is a political non-starter in America. There are no easy answers.