Fair enough. The effect will lessen, but it will still be there. Even if he can convince half of those who right now wouldn't to just consider voting for a socialist, he still is down a quarter of the vote from the get-go, and convincing half is a stretch.
In other election years that might be right, but everything is up in the air this year because of years of frustration with the Republican and Democratic establishment. So Socialist Independent might be a big selling point to a lot of people who are sick and tired of the process. These vast swaths of the electorate who are dissatisfied don't really know or care about those labels at the end of the day - I mean Capitalism itself only polls 3% higher than Socialism, for instance - what they want is change from establishment politics. And as soon as they hear Bernie speak, when they figure out that lots of their fellow Americans love this guy, when they hear that lots of Republicans are going to vote for him, that label isn't going to be the negative you think it will.
It doesn't show that Americans agree with his proposed solutions.
Infrastructure Spending: I think you're referring to the Fox News poll which poses the question "do you want additional infrastructure spending" with the phrase "if your taxes go up". First, that's a pretty cheap polling trick to phrase a question in a way to get the result you want, which isn't all that surprising coming from Fox. Second, that's not what Bernie's solution is. He wants to tax the upper income brackets. Given that vast majorities of Americans support higher taxes on the rich, the question should have been "would you support this proposal if it were funded by an increase in taxes on the top 10%"? Without the misleading and false Fox News qualification, support is around 75%.
Trade restrictions: Americans agree with Sanders (and Trump), and are against the ideas that people like Jeb, Hillary and Obama have been promoting the last 30 years. That's a policy proposal, not a problem.
Raising minimum wage: 60% support. Another policy proposal.
I could go on and on, or just go and read the article in full. WaPo didn't just make up the title "Bernie Sanders says Americans back his agenda — and he’s mostly right" based on data that doesn't support that point.