His reasoning was complex but a lot of it revolved around the simple fact that as you get more people publishing, the intellectual quality of the average published work goes down.
I'm not ready to roll back the printing press, but in retrospect the digital era has proven him right about this. For instance his position kind of predicts Eternal September - the easier it gets to post online, the more numbskulls you have doing it. Microblogging is the ultimate expression of this and frequently the content you find on microblogging platforms is the absolute worst hot takes and generally the most vile stuff the moderation rules will permit because shock value generates impressions. It's every idiot on earth competing to be as flagrant and base as possible.
We usually hold up free speech as a virtue in Western societies and there are a lot of good reasons for that, but I'm increasingly inclined to treat microblogging less like publishing, and more like alcohol/tobacco/gambling, like it's something people do but they know it's not good for them, they do it anyway because it's addictive and easy.