Well if your point is "people who are bad at writing screenplays shouldn't write screenplays" I agree.Sure, but the rest who are good at directing, bad at writing scripts probably shouldn't emulate them.
But your post specifically calls out directors writing scripts as the reason for bad movies and I don't think that holds up to any scrutiny. Lots of the best movies in recent years were written by the director, and lots of the worst movies were written by full time writers.