It has its strengths as well. The idea of pledged delegates being distributed on the basis of the popular voter is better than the Republican plan which can award all the delegates even if a candidate gets less than 50% of the vote. Super delegates do not mean anything. Most of the super delegates as well as the DNC supported Clinton in 2008. However when Obama started winning states, many of the super-delegates who had initially supported Clinton defected to Obama. In 2016, Clinton got 55% of the primary vote.
Sanders had no chance because he had no plan for winning the nomination after the first 4 contests. Sanders got clobbered in the south, winning only 26% in SC, 19% in Louisiana and Mississippi, and 33% in Texas and Florida. Clinton turned a handful of southern states that went to Obama in 2008 while Sanders was only able to turn Oklahoma. The DNC does not give any money to candidates. It was fair as 55% of primary voters voted for Clinton.
It's not exactly as you say .... super delegates (which is different than pledged delegates) are free to vote for whomever they choose. They are NOT assigned - your claim of requiring 50% is false. The supposed 50% rule is only used when the election is decided before they cast their votes at the convention. It is the custom to do a unanimous proclamation which makes the winner look like he/she has a greater mandate. In the case of both candidates being less than 50%, they are free to vote for whichever they wish.
15% of all delegates are super delegates - out of the 4,763 delegates, 715 were super delegates.
Since we know that that 30 million voters voted ... each delegate (on average) represents 6,298 voters.
Now, the super delegates represent one voter - themselves. Their votes could have canceled about 4.5 million voters (6,298 x 715). The margin between Sanders and Clinton was only about 3.7 million voters - less than the offset of super delegates. So, it is conceivable that, if all 715 super delegates had chosen to vote for Sanders, it would have ignored the will of the Democrat voter.
THAT is the flaw in the system. Nobody is saying it threw this election one way or the others - but it COULD have.
Unquestionably, if the Republicans had used the same system, the antipathy toward Trump within the rank-and-file Republican Party hierarchy was such that he would not have won the nomination, and the will of the Republican voter would have been subverted. Fortunately, the Republicans don't use super delegates, and the people spoke.