The reference to Franklin D. Roosevelt was inadvertently revealing. Roosevelt adopted reform policies, including many of those suggested by the social democrats of his day such as Norman Thomas. He was no socialist, but rather a clever and conscious bourgeois politician who enacted limited reforms in a deliberate effort to save the capitalist system.
Sanders and Ocasio-Cortez likewise seek to save the capitalist system, but under conditions where no such reforms are possible. The American ruling class no longer dominates the world economy, but is beset by powerful rivals in both Europe and Asia. It is pouring resources into the military to prepare for world war. And at home, even the most modest measures run up against the intransigent opposition of the super-rich, who control both parties and demand even greater wealth for themselves at the expense of working people.