Why do art historians and critics believe that modern art has ended?

I believe that the main reason the art critics and historians believe that modern art has ended is because the peak of its abstract expressionism has long since passed. Modern art was created from about 1860-1970 and included much experimentation such as Picasso in say, showing faces turned at an angle where only one side would be visible but showing the other side as well, or Emil Carr going past realistic paintings of forests and totem poles to more abstract forms of them, especially the forests to give a strong impression of the feelings evoked by the subject matter.