But you can't "improve" the ACA into single payer. You can only get single payer by starting over from scratch.
The pathetic thing is that a public option, the original plan, could quite easily be improved into single payer. And despite the largest Democratic majorities in both the house and senate for several decades, with a sparkly new progressive Democratic president that campaigned primarly on health care reform, they couldn't pass it. It seems the Democratic party has something to learn from the Republican party on how to get shit done.