Totally agree. Impeachment serves as a check on a sitting president by a co-equal branch of government, the legislative branch. Trump is now a private citizen and therefore, as you note, this whole charade becomes moot.
I never thought the branches are co-equal. Congress controls the POTUS, if they want to. We just saw four years of Mitch controlling who Trump chose for his Admin. Congress can impeach Justices, can change the number of Justices, or their jurisdiction. Legislative is the lead branch, then Executive, then Judiciary. Trump just found out the POTUS is not the most powerful man in the World.