There are several significant takeaways in "A Plea for Time," but they generally all lead to powerful statements regarding communication near the end of the essay. I would argue that the main argument is that modern civilization stresses an immediacy in communication that has depleted society of its concepts of community.
Innis argues that modern society has become "paralyzed" by our inability to maintain an interest for any duration of time. We are also "under the spell" of the media around us. The printing industry, radio, and television are all delivered with a great and convincing sense of realism—which makes it increasingly likely to also offer a greater "possibility of delusion." Innis points to the success of the German media, which compelled the German populace to believe in the superiority of the German people. This demonstrates a "fallacy of misplaced concreteness" that exists through our connections to media.
People therefore entertain superficial thinking, even in their educational pursuits. Universities push courses which are likely to make more money for the school. Students are tested in their ability to receive information instead of for their ability to sustain meaningful discourse. Language is simplified so that more people will be able to read and write a "simpler language." Thus "education is a thing of which only the few are capable."
Unity therefore suffers as universities become obsessed with funding themselves, proving that even higher education is a superficial endeavor due to an ever-pressing need to simplify communication.