"Driving with the parking brake on" is a great analogy. A lot of us love Bernie and his ideas. We just don't think much of his strategy to win. And you DO have to win to implement any changes. You can preach in the forest and the trees might agree with your positions and think very highly of you. Unfortunately, they don't vote so nothing comes of it.
We don't have a parliamentary system, so a Third Party affiliation is very unlikely to win much more than a House or Senate seat. The way to win the presidency, is to join one of the two parties and then rise up through the ranks. If you have stellar qualities, you will rise fast. You can have extreme ideas. Both parties have their extremists. Look at Rand Paul: Extreme libertarian, but he always runs (and wins) as a Republican.
You can't drive the Democratic Party to the left unless you are in the Party. Standing outside and criticizing it does very little.