Bernie Sanders was the very definition of “straw man”.
And like the scarecrow set up in the cornfields, totally ineffective.
Enter Bernie, the fake opposition. Bernie Sanders is perfect for the DNC. He's visible enough thanks to a loyal cult following, yet has zero chance of being nominated, because outside his cult of college kid fans, everyone knows he's unelectable in a general election. And just in case Sanders would get high enough in the polls to embarrass Hillary in early primaries, he gets told to tone down his opposition - i.e. Bernie pre-emptively defending Hillary about the email server scandal (no doubt on orders from DNC higher-ups), Bernie being put in his place about access to Democratic donor information, etc.
Another example of how the DNC has choreographed Hillary's coronation is Joe Biden's long "decision" not to run. It wasn't his decision, it was the DNC's. He was allowed to consider running early on as a pinch-hitter if Hillary's scandals exploded. Since they didn't, Biden was out.
The RNC tried to same stunt with the intent of coronating Jeb, but fortunately things didn't work out as smoothly for the GOP establishment as they did for the Democratic Party.