The argument I buy: this is something you can plausibly get 50 votes for, it will be a material benefit to millions of people, it will be almost impossible politically to roll back.
What I don't buy: we can pass legislation to build on that after the midterms.
We're in a world dominated by negative partisanship where midterm backlash is inevitable and Republicans in Congress are never going to hand a Democratic president 1) a big legislative win that 2) vastly expands the government's role in providing health insurance. Maybe the public option/Medicare expansion puts us on a glide path to single payer, but after the Republican party's decade-long fight against the ACA I'm not gonna be too sanguine about that.