It's just as feasible that Democrats control all three, and it's happened before. The Democratic party had complete control over both Houses and the presidency

It's just as feasible that Democrats control all three, and it's happened before. The Democratic party had complete control over both Houses and the presidency from 1933 to 1947, 1949–1953, and 1961–1969; 26 years of full control of the legislative and executive branches thanks to the New Deal Coalition. By 1939, FDR had formed a more liberal court through his appointments, and that lasted a while through this Democratic control of the branches into the 1950s. As the pendulum swings, so does policy. It would be more difficult IMHO if there were more than two parties.