Marshall McLuhan coined the phrase “the medium is the message” to draw attention to the heretofore underestimated importance of media. Trains can serve as a simple illustration of what he meant. It wasn't the cargo in the trains that was of prime significance to society. It was the fact of trains.
But now things have changed. Trains were the arteries of change at one time. But those arteries have grown in number and diameter. Now it's not just trains but TV, radio, internet, smart phones, and too many other examples of media to mention.
Media has become a jungle of arteries that are so thick that individuals can't negotiate them or penetrate them. We are hamstrung by media. And these media transmit useless mediocrity at best and toxic folly at worst.
Now media isn't just the message; media has become the aorta of survival. So now the idea that “the medium is the message” has become obsolete. Now the message that media transmits is all important!
So as innovative as Marshall McLuhan was as a thinker, this is something that I doubt he could understand to the degree you and I can understand it.
Of course I probably wouldn't understand it at all if not for Marshall McLuhan.