I'm not sure I agree with you. Cohen's guilty plea certainly doesn't prove that Trump is guilty of the same crime... but taken with the statement that the payments were made at Trump's direction, I would say that it can have a legal impact on Trump - It gives the prosecution something that they can charge the president with directly, even if they don't have enough evidence to get a conviction out of it. To date, I think this would be the first thing they can directly charge Trump with, isn't it?
I mean, they may not charge him (although I don't know why they wouldn't), but they could, couldn't they?