That easy.
Democrats passed the ACA by reconciliation .. which essentially means that they can pass anything they want regardless of republican objections.
Medicare is a long-existing program, thus extending it to all Americans would have been easy .. infrastructure, functionality, and user-understanding already in place.
One of the most popular programs in American history.
No mandate.
Employers taken out of the healthcare equation.
AND, the question of constitutionality would have never been asked.
Conservatives and Liberals Agree: Medicare for All Would Be Constitutional
http://www.huffingtonpost.com/miles-mogulescu/medicare-for-all_b_1400712.html
They could have done far better than the ACA .. and the reason they didn't .. Barack Obama
He is a corporatist .. not a democrat.