That depends on your point of view. Clearly the party has a lot of control over who they nominate, unlike the Republicans, who lost complete control over their nomination process.

Perhaps parties shouldn't have such tight control over things; I certainly would have strongly preferred Sanders as the Democratic nominee. Still, it's a party. Technically it is up to the party to decide who they nominate. That they use primaries to make this decision is certainly nice and democratic of them, but it's not a legal requirement, and it wasn't always like that.

The thing that's really broken is the entire political system that only offers these two parties as realistic options. But the Republican party has also become really dysfunctional over the past decade or so, where representatives and senators basically refuse to do their job and work to undermine the country by their political gamesmanship.