The overspeed condition was the proximate cause of the accident because it increased aerodynamic forces on the horizontal stabilizer that prevented the crew from manually overcoming the AND (aircraft nose-down) trim applied by the MCAS and also resulted in a terminal dive.
Turns out that it's hard to build an aircraft that is resistant to abuse by pilots with grossly inadequate flying skills.
Maybe Boeing should have taken the Airbus approach and have three independent flight computers vote on whether the pilot is an idiot. Oh, no, wait! They keep managing to crash those as well.