"No such letter will ever be published. Why?"

Sure, I can tell you why.  Because you included this in your imaginary letter: "In our view, further continuance of a long record of proven failure is even more worrisome than reckless rhetoric about future policy."

Clinton is a seriously flawed candidate, but she in no way approaches the train wreck that is Donald Trump. No one outside of the fringe right is going to sign onto a letter supporting Trump over Clinton.  If the right wanted to convince us to vote for their candidate, they would have offered one that didn't have serious mental health and ethical issues.