The pamphleteer Thomas Paine and the rabble-rousing Samuel Adams gave way to the more moderate George Washington, Thomas Jefferson, John Adams and James Madison.
In the middle of the 19th century, radicals like John Brown and purists like Horace Greeley gave way to the incrementalist Abraham Lincoln.
In the Progressive era, the agitation of Ida Tarbell, Upton Sinclair, Jane Addams and "Mother" Jones led to the trust-busting and reformist Theodore Roosevelt and fellow-progressives William Howard Taft and Woodrow Wilson.
The five-time Democratic Socialist candidate for president, Eugene Debs, laid the groundwork for President Franklin Delano Roosevelt and the New Deal.
Civil rights activists like Martin Luther King were the impetus for the work of John F. Kennedy, continued by Lyndon B. Johnson and the Civil Rights Act of 1964.
At a certain point, idealists and radicals give way to pragmatic and moderate politicians to make their goals a reality.
In light of our history, it's not too difficult to realize that Bernie Sanders needs a Joe Biden to accomplish what Sanders envisions. Sanders knows it and endorsed Biden. With a Democratic presidential victory, Sanders and other progressives will pressure Biden and Harris to achieve their longed-for agenda.