One of the most notable things about this election has been how strongly disliked the two main candidates were. They famously have the lowest favorability ratings of any candidates in recent history. Putting aside any corruption and favoritism from within the parties for a minute, it seems like the primary system really sets the parties up to elect poor general election candidates.
For example, take a look at the states where Clinton performed best in the primaries. They're Mississippi, Alabama, South Carolina, Georgia, Louisiana, Arkansas, and Texas. All of these states were decisive victories for Trump in the general election and from a perspective purely of wanting to win, it doesn't make sense for them to be so important in the primaries. At the same time, Bernie won a number of states in the primary that ended up being important swing states (e.g Michigan, Wisconsin, New Hampshire, Maine, Minnesota, Colorado). From a strategic standpoint, it would seem to make sense to attach a higher weigh to those victories (the same is also true for Florida, Ohio, and some other states where he lost).
None of this necessarily means that Bernie would have won, I just think that it seems as though the primary process is not very well designed for choosing the best general election candidates. This is even more true if you consider what a large proportion of states have closed primaries that specifically exclude independents. It seems obvious that they will be far less likely to vote along party lines in a general election but, in many states, they're given no say in the primary process. I can understand why the DNC can't just say, "sorry, but the deep south doesn't count and neither does New York or California," but opening up the primaries seems like a perfectly reasonable strategy to put forward stronger general election candidates.
I don't really have a broader point beyond that, it just seems like the parties really set themselves up to produce such, to put it gently, sub-optimal candidates.