Throwing out another game-theory possibility of what's going on re: impeachment and the next election.
The both the Dems and GOP know that Dotard is going to get killed in 2020. But the GOP knows that trying to primary him is a long shot and probably wouldn't happen. So the best chance for the GOP in 2020 is for him to be impeached + senate conviction so they can run someone else.
The Dems know this too, and they would rather secure 2020 than kick Trump out for one year. So their strategy is to drag out all of the shit on Trump and continually remind everyone that he is an impeachable shitstain, even if they do not go through with it. That solidifies 2020 for them, whereas impeachment has a lot of unknowns associated with it: will it blow back on them in the court of public opinion? Will the senate convict, and GOP unite behind a centrist candidate to counter the huge, divided field of democratic candidates? etc.
If there is another GOP candidate, he/she will probably lose too, but at least they have a chance. With Trump they have no chance.