Either you've forgotten or weren't old enough to remember the MSM in the 80's... And there was no real "alternative" media to get information from then and certainly no Internet. Thinking about it, CNN's Crossfire was one of the few shows I watched back then because you could get alternative information. Think about that for a minute or two...
Even earlier -- the 60s/early 70s, during the Vietnam War. Back then, the bias was a bit more subtle, but it was there. It helped turn public opinion against the war. And the war went onh during Johnson's term as well as Nixon's. The president's political party didn't matter.
Anarchists, possibly communists, determined to destroy America began to infiltrate every aspect of our lives, including the media. Most of us who were around back then didn't notice. Heck, we watched Walter Cronkite, billed as the most trusted man in news, or some such, every night. But when I view video clips of Cronkite from the Vietnam years today, his bias is now more obvious. Probably because in recent years, with so many ways of checking and rechecking every news story, we are now more attuned to the BS.
Anyway, I still think Trump is wrong to blow off this dinner. It would show that he won't be cowed. It would show he has the courage to enter into the lions den. And he could have gotten off some manure to sling at the media jackals in the process.
That would have been "winning."