This chart by TV Newser says it all:
Not only did ABC barely beat the Fox News debate (that Trump boycotted) in overall ratings, they also lost to Fox News in the very important 25-54 ratings demo by a smidgen.
Of course this was a Saturday night debate versus a Tuesday night debate by Fox News, so that may have been part of it.
But I also think this goes a long way to dispelling the laughable notion that if Trump had participated in the Fox News debate it would have had 24 million viewers like it did the first time. Trump was in this debate and it was only marginally better than Fox News overall.