I don't think Manafort hurts trump at all. He nosed his way into the campaign to help his consulting business that no longer had any clients. He was gone once his Ukraine connections came to light. I don't see how Cohen hurts Trump. Setting up NDA's during a campaign may look bad but they aren't illegal if done properly. If they took the cash out of campaign contributions, that would be illegal. There was some precedent set at the John Edwards trial when he was found not guilty, but maybe the DOJ wants to go back and revisit that issue.
DOJ included the two campaign finance charges to link Trump to Cohen and a crime knowing full well that these charges will not be tested in court. Cohen has copped a plea bargain deal ensuring that there will be no trial. If Trump needed any more convincing that he should not sit down with the Mueller team, this Cohen deal should put that to rest.