What did both George Washington and Thomas Jefferson believe would happen to slavery sometime in the early 1800s?

Slavery will end

Explanation:

The two prominent Founding Fathers, and former United States Presidents, Thomas Jefferson and George Washington, were both advocates and pioneers in the movement to end slavery in their State and in the nation. In there bid to end slavery, both Jefferson and Washington sought to liberalize the State's slavery laws to make it easier to free slaves.

With the first federal racial civil rights law in America been passed on August 7, 1789, and fully signed by President George Washington, which prohibited slavery in any new State that might seek to enter the Union. Thus, slavery was prohibited in all the American territories held at the time; and it was because of this law, signed by President George Washington, that Ohio, Indiana, Illinois, Michigan, Minnesota, and Wisconsin all prohibited slavery.

Hence, they believe that, with their constant struggle and ability to convince others, slavery will end in United States by 1800s.