Sanders, just like Trump, is running as a third-party candidate within a major party. This isn’t the first time Sanders has recognized the benefits of working within established Democratic power channels in his career. Sanders has been more effective in Congress because he has caucused with the Democrats and his Senate seat has become more secure since national Democrats recognized him as (basically) one of them rather than someone to replace with a real Democrat.
The examples go back much further than that. Ron Paul had a bigger impact on the national debate finishing a distant fourth in the Republican primaries in 2008 and 2012 than he did winning the Libertarian Party nomination in 1988.
Gary Johnson, the 2012 Libertarian nominee, was a success story because he won more than 1 million votes in that election. But Ron Paul won more than twice that in the GOP primaries alone that year. And as a Republican, Johnson served two terms as governor of New Mexico. He was not just a rounding error in a contest between Barack Obama and Mitt Romney.
Didn’t Howard Dean do more to move the Democratic Party to the left than Nader, who helped cost the Democrats the election in 2000? Didn’t Pat Robertson’s 1988 presidential campaign do more to mobilize conservative Christian voters than the Constitution Party has done in more than a quarter century since then?