If Democrats want to avoid a replay of 1968 or 1980, with a Donald Trump or Ted Cruz winning the White House, they are going to have to lower the temperature in the room and find ways to remind their supporters that the cause of party is more important than the imperative to be right.
The current primary systems are completely different from 1968 (where Humphrey nearly came back to win and faced George Wallace taking away key Democratic states remember) AND 1980.
Kennedy's attempt to free the delegates led to the superdelegates. It was arguably this move, combined with Kennedy's withdrawal speech that may have hurt Carter more than any dumb handshake. Not to mention John Anderson's bid. And the Carter campaigns all around incompetency and assumption that Reagan had no shot.
1968 had only a handful of primaries, the bulk of delegates were controlled by the political machines and thus even had RFK not died it would have been tough for him or McCarthy to stop Humphrey at the convention.
Four years later, they attempted to derail McGovern but the new primary system made it so they were able to ward off the attempts. (Hunter S. Thompson's work is oddly one of the few extensive (if biased) records of the Humphrey/machine convention attempt to unseat the McGovern elected delegates.)
The Sanders people, like the Hillary people in 2008, will know ahead of time that they were defeated by the process, even if they consider it unfavorable to them. There's no convention fight to be had. Remember, Hillary almost didn't release her delegates at the 2008 Convention, but was persuaded to halt the roll call. The PUMAs were ultimately insignificant, as was everything prior to Johm McCain's STOP THE DEBATES, WE'RE ALL GOING TO DIE, WE NEED TO HEAD TO WASHINGTON IMMEDIATELY AND HOLD MEETINGS FOR SOME REASON abject meltdown, while Obama played it cool.
The GOP is in a different situation because 1.) Donald Trump is involved and B.) No candidate may enter with an obvious path to a majority and iii.) Donald Trump is involved. Also, Ted Cruz is the alternative.
One could expect that any gains Trump gets from anti-establishment/anti-trade/etc. voters who backed Sanders to be countered by Republican-leans who won't accept Trump. And Cruz has none of the crossover potential considering his standard conservative positions across the board.
Also, one shouldn't confuse the Sanders fanboys for the bulk of Sanders' support, anymore than one should confuse YAS QUEEN as Hillary's support. There's a lot of simply anti-Clinton, anti-politics-as-usual, etc. voting there. Maybe even more than there is true "left-wing" voting. It's a harmless protest vote against an believed inevitability. Most of those people will stick to voting D in November because it's less that they hate Hillary than they're just unenthusiastic about her but she's still clearly better than Trump/Cruz.
