I think you are pretty accurate in how the election is going to go, regardless of who wins the D nomination. But there is a purpose in the D’s continuing to attack in this way:
It convinces some of the soft R support to stay home on election day. A vote not cast for Mr. Trump is almost as good as a vote cast towards the D way.
Political parties have been playing this game for decades. Negative campaigning does work in keeping certain voters away from the voting stations.
I am surprised that many political writers on Medium don’t realize this.
In the meantime, I would wager that back-room strategists in the various D campaign offices are devising ways to go after the soft R support.