Byron York has a column in USA Today suggesting the Democrats abandon their system of proportional delegate allocation and adopt a winner-take-all-system. York says, "Yes, winner-take-all can be rough. But at least it produces a winner."
The effects York identifies--the protracted campaign and the possibility of a popular-vote winner losing the delegate count--are real enough. But this isn't an indictment of proportional delegate allocation. The campaign is so protracted mainly because of the existence of superdelegates: If there were only pledged delegates to worry about, the contest would already be over, and would functionally have been over in February. The remedy here would be simply to get rid of superdelegates. What's more, if the concern is that things are dragging on too long (I'm not all that worried about this, but others are), the party could simply require states to hold their contests by, say, the end of April.
Similarly, the possibility of a popular vote–delegate count split arises not as a result of proportional allocation, but because votes in caucus states count for more than those in primary states, since turnout is lower. If this is a major concern (again, I don't really think it is, but assuming it for the sake of argument), states shouldn't be allowed to hold caucuses. It's also true that the specifics of the system the Democrats use, where delegates are awarded proportionally by congressional district rather than statewide, don't make much sense and add another layer of distortion to the process. I'd like to get rid of district-based delegate selection. But that, too, isn't a problem with proportional allocation as such.
The fact of the matter is that winner-take-all produces a winner so quickly primarily because it ignores the voices of large minorities who prefer one of the losing candidates. In a primary, where you often have multiple candidates and the "winner" sometimes receives only a third of the vote, that's a major problem. There are plenty of ways to fix the ostensible flaws of Democratic primary system without abandoning the essentially sound premise that in a primary election, the share of the vote a candidate receives in a state is the most accurate reflection of the will of the electorate.