For nearly 10 years I have referenced this thread in various forums I've moderated. While I never entirely agreed with every aspect, it has mostly held up well as a lodestar over the years.
And now, with the benefit of enough sequential observation over time, I am comfortable describing what I believe is a major hidden assumption, and thereby weakness, in this entire argument:
For the concept of "walled gardens" relating to online communities to succeed and thrive, there must exist an overlay of credibly alternative platforms. Or, more directly, there must exist fair and healthy competition among and within the media upon which the discussion are taking place.
This argument was created largely before the "sciences" of social media were refined. Today, we are living with the consequences of intersecting sciences of human psychology, sociology, computer science, statistics and mathematics. Dense, voluminous texts have been penned which describe precise models for determining how to create, manage and extract-value from "online communities". Some of those models even go so far as to involve manipulation of human physiological responses -- i.e., intentional mechanisms within the platforms designed specifically to trigger release of chemicals in the human brain.
There exists an analogue for this maturity curve within television advertising. As both the medium and techniques matured, the need to evaluate how we, as a society, managed its impact fundamentally changed.
Today, in late 2018, there effectively exists no credible "public town square" whereby free speech exists as it was intended (within the intention of the US Constitution). What exists in its place is a de facto oligopoly of media companies posing as tech companies who have divided up the horizontal market and who exercise overwhelming "market power" (as in HSR power) over would-be competitors. Those competitors are then relegated to competing as "free speech purists", which leads to the traps outlined by the original argument: a cesspool of fools and insults.
This situation allows the dominant players to then, in turn, point to the worst aspects of their would-be competitors whenever they feel threatened by them -- or are otherwise politically or economically motivated. Using catch phrases like, "hate speech" or whatever "ism" catches the gestalt, the oligopolists then pressure the supply-chain of would-be competitors, forcing them out of business. They eliminate their ability to process payments. They cut off their upstream bandwidth providers. They remove their ability to be routed or resolved. They eliminate all possibility of collecting advertising revenues.
And they do all this with the virtuous facade that they, the incumbent giants, are safeguarding a "well kept garden". All while conveniently forgetting that they only rose to such dominance by exploiting the very freedom of speech -- including an early tolerance for the opinions they now so self-righteously claim to oppose.
There are various academic ways to describe this type of situation. But the solution is the same: largely unrestricted free speech and a renewed focus on one's personal responsibility in selecting herself in or out of conversations.