Election fraud is much easier to sell to the electorate when it occurs blatantly in full view. If Democrats held honest and open primaries that allowed the people to choose their candidates they wouldn't be so open to accusations in the general elections.
Whatever might be said about Trump, he was not the choice of the GOP leadership or "ordained" by any interest groups. He came into power by way of appealing to the voters who were desperate for a strong leader. Their intellect, and perhaps their sanity, can be questioned in that choice, but not the process.
The next "Trump" is likely to be more articulate and better mannered, and that should be a worry for any Dem who is concerned about the future of our country. If the DNC continues to validate the accusations made against it, the candidates they "install" will have an uphill battle before a general election ballot is cast.