Voltaire pretty well killed the idea that this is the best of all possible worlds in Candide!
In 1776, the founding fathers demonstrated that they did not believe that they then lived in the best of all world by revolting against their British government. The founding fathers created the Articles of Confederation and Perpetual Union to make things better, and soon thereafter created the Constitution recognizing that the Articles did not make this the best of all possible nations. President Lincoln took office realizing that slavery was not a condition compatible with the best of all possible worlds, but that Civil War was even worse than slavery. All the presidential candidates -- Democrat, Republican and Bull Moose -- in the election of 1912 ran on platforms that they could make things better, and then the world got into World War I, which was not better. FDR took office early in the Depression, realizing that his job was to make things better. At the end of World War II, people all over the world recognized that they had not been living in the best of all possible worlds, and sought means to make the world better. In my lifetime, people who got the USA out of Vietnam, who promoted civil rights, women's rights, and the rights of the disabled helped make the country better.
Of course, change can be for the worse, and society needs conservatives to see we don't throw out the good with the bad. Still, we all live better, happier lives than did the founding fathers because so many people have tried so hard to make things better!