To answer your last question first - why does Clinton need the Sanders supporters? First, in a best-case scenario, Sanders supporters matter down-ballot (and in the future), not that the DLC (which has basically eaten the DNC) has ever been particularly concerned about anything but the presidency (and the present); while Hillary can probably win the Electoral College with her supporters + moderates/independents/anti-Trumpers (including Republicans), many of those voters will turn right around and vote R all the rest of the way down the ballot.
So you end up with Clinton in the White House and Republican control everywhere else - Clinton will be fine with that, the establishment will be fine with it, non-insane Republicans will be fine with it... but for anyone actually on the left, or interested in change of any sort, it's a disaster (and one that is just begging for blowback down the road - think of it as filling the cracks in the party with explosives to be detonated at a future date).
Second, in less-favorable scenarios for Clinton: if Trump manages to appeal to more independents / moderate voters somehow (I'm not certain how myself - maybe via a brilliant VP pick - but I'm unwilling to write-off the possibility), then Clinton is going to need that strong anti-Trump, anti-conservative bloc, and need it badly. If she has instead alienated the traditional Democratic voters to the point where they stay home, vote 3rd party, or even vote for Trump, then she's toast.
The idea that the "Clinton / Establishment hold all the cards" is true, from the perspective of the
nomination - the problem is that they are treating that a though it means the pro-Sanders left has no alternative but to vote for Clinton in the general; and that is only true to a certain extent. And, again, from the perspective of Sander's supporters, they're being so blatant about the "you have to vote for Clinton, haha" aspect of it that they're on the edge of pushing significant numbers of these voters into abandoning the party, or at least the Clinton wing of it.
Look at something like this piece from Counterpunch:
"Clinton Does Best Where Voting Machines Flunk Hacking Tests: Hillary Clinton vs. Bernie Sanders Election Fraud Allegations". Regardless of its ultimate accuracy, this is the headspace that a non-zero segement of likely D voters are in - adding state convention shenanigans to it just pisses them off more. For that matter, how is a Democratic National Convention that looks like a riot going to help Clinton when compared with Trump's WWE show?
But more than the present election, this is about the future - barring a game-changer, Clinton can likely force a win in November, even with alienating the Sanders supporters; but doing so will be the end of the traditional Democratic coalition. Clinton is moving in the same direction as the Republicans, courting older voters at the cost of younger ones - that's a net losing strategy in the long term.
But I'm starting to repeat myself, so I'll do it one more time and call it a night: you're right that Clinton likely won't
need the Sanders voters to win, but it looks to me like she's alienating them
needlessly in ways that it will be difficult for the party to recover from in the future, or for her to recover from if she suddenly finds that she does need them in November, it will be too late. (And I'd really hate to see President Trump arrive because the Democratic nominee stupidly burned the party's bridges with the left.) It might all end for the best though - if, 12 years down the road, we have a right-wing socially conservative populist party, a centrist pro-business party, and a socially and economically progressive left-wing party, I'd prefer that to our present system.