His previous work isn't on trial here though. His article outlined several reasons that he sees as evidence that liberalism is wearing out is welcome with the American people. IThe fact that liberal media doesn't do well is just one of them, though in his piece, he equated this with the canary in the coal mine.
I would certainly think that if Fox News conservative commentators and conservative talk radio hadn't been able to attract audience and post successful numbers, we should at least consider that there is little interest or enthusiasm for conservative thought and commentary.
Another signal he saw as noteworthy were the results of the 2010, 2012, and 2014 elections, each suggesting increasing disatisfaction with the liberalism of the current administration.
And his third point was in this paragraph:
Meanwhile, the cultural left has disengaged from mainstream political arguments, preferring instead the comforts of identity-politics argy-bargy. You judge political movements not by their manifestos but by where they put their passion. And on the left these days, the only things that arouse passion are arguments about race and gender.
And if you read back through this thread you will see the singlemost often mentioned issue used to defend liberalism is all the states that have legalized gay marriage. (Of course there are numerous conservatives and libertarians who also don't oppose gay marriage, but let's not worry about that right now.) The point is, if you don't have anything other than gay marriage to point to for the liberal manifesto, Goldberg might be right that the movement is losing favor.
So far several have objected to the thesis of the thread and/or Jonah Goldberg and/or that they can't comment without having terms defined to their satisfaction. There have been a couple of poll results, nothing really recent, showing liberalism ticking up a bit as an I.D.
I don't know whether Goldberg has really made his case. But unless I missed the post, I don't know that anybody has really rebutted it either.