You do not have to argue that Bernie Sanders will pass his single payer plan, or even any single payer plan, to insist that a vote for Sanders is a vote for single payer and a vote for Clinton is a vote against it.
Vote for Clinton and you endorse Clinton's monstrous right-wing arguments against single-payer. You affirm, as she proposes, that we should take into consideration the tax hikes you get from single-payer, but not savings on insurance premiums. You affirm, as she proposes, that the federal government cannot possibly design a single-payer system that delegates various administrative and funding responsibilities to the states, even with a federal fallback in place. And worst of all, per Chelsea's grotesque attack, you affirm that people should fear losing the meager system currently in place instead of aspiring for something better.
Vote for Clinton, and you guarantee that we will spend the next eight years talking about single payer as a political impossibility, if we talk about it at all. The left will have zero stake in advocating it and fighting for it, because they know that even the Democratic White House will oppose it. The fight will become re-centered on the question of whether even tweaks to Obamacare are feasible, or whether we should simply roll back the progress we've made. Our entire media apparatus will spend the next four-to-eight years exploring small-bore technocratic mods to the ACA in pornographic detail. Our left think tanks will roll out dull graphs hyping a potential a 4 degree change in the trajectory of the cost-curve, while Cato spams Capitol Hill with terrifying graphs of insane economic catastrophes.
Vote for Clinton, vote against single-payer, and you will eliminate a major incentive for the left to vote in 2018 and 2020. Democrats will have little stake in trying to win back control of Congress, or (crucially) to win the next redistricting war, because they will know that the only immediate payoff would be the marginal changes Clinton has put on the table.
Vote for Clinton, vote against single-payer, and when the next recession hits (quite possibly during her term in office) you will deal a massive blow against public confidence in liberal Democratic governance. You will have a jump in unemployment, you will have all kinds of economic duress, and you will suddenly have hordes of voters who cannot afford their bronze plan premiums. Obamacare will be worthless to them, and they will have no reason to hope for things to get any better under Democrats, because the one thing that could help them - free health care - is now off the table.