The first point is above my pay grade and I am not interested in arguing it (roughly I would say this is dumb, as I think a president shooting a child on the White House lawn should be arrested and prosecuted like everyone else, and this logically leads to other places), but we can't have a system where the first is accepted as true and the bar for the appropriate remedy of impeachment is set so high that it is never invoked unless there is a Democratic president and a Republican Congress.
Of course (well, probably) Trump isn't going to be convicted in the Senate and removed from office. Bill Clinton wasn't going to be either. That isn't the only point. Nor is it convincing David Brooks that it is the right course of action. Nor is it maximizing 2020 election chances, whatever course of action would do that. It's about saying, "this is bad, mmmkay?"*
*With Bill Clinton what they were saying was, "Democrats are not allowed to be president," which is also bad, and the rest of the country understood that and would have continued to if the Democrats hadn't been idiots.