“Complete” freedom of expression is too extreme. We don’t know whether people in the past had complete or less than complete freedom of expression. The author is simply arguing that we should ensure that today Internet users have at least as much freedom of expression, whether that was less than complete or complete.

“All” and “same” are too extreme/specific. Even if some citizens have slightly different levels of access to the Internet, that doesn’t undermine the idea that the Internet serves the role of a place to discuss issues, nor does it undermine the idea that Internet users should have at least as much freedom as people did in the public square.

Necessary, because if it were not true — if a public forum CANNOT lose effectiveness as a tool of democracy even if participants can’t discuss issues freely — then we have no reason from the premises to believe freedom of expression is important for serving the role of a public square. The Internet could be just as effective a place for people to discuss issues even if there were restrictions on freedom of expression. (C) is necessary in order for the author to value freedom of expression for the purpose of serving the role of a public square.

The proportion of specific issues discussed isn’t necessary to the argument. What matters is that people can discuss the important issues of the day; whether they spend more time discussing frivolous issues doesn’t change the fact that it can still allow people to discuss important issues.

Not necessary, because even if it were not true — if there are some other public forums that are important tools of democracy — that doesn’t undermine the position that the Internet plays the role of a public square and that we should ensure that there’s as much freedom of expression there as there was in the public square. The Internet can still be important enough to need protection of freedom of expression, even if there are other places people can discuss issues. The author might even argue that those other places should have as much freedom of expression as the public square, too.