“Mr. McConnell has a thin margin. If Democrats stick together and four Republicans defect, Mr. McConnell could lose control of the proceedings.” That is, a caucus of 3 or 4 Republicans (or a bipartisan one of 6 senators) could ensure a full and fair trial.
The Senate is supposed to render “impartial justice” in an impeachment trial. Mitch McConnell is letting Trump plan his own trial.