What Trumps' constant attacks on the national media do is inflame his die-hard supporters, like he's doing with the NFL so that they react with anger.
That type of anger makes people turn away from "mainstream news" and embrace conspiracy theories from black helicopter groups and the Fox and Friends state-sponsored propaganda have been peddling.
This is detrimental to a free society, but by his reactions to the press, free is something Trump would love to do without.