So the practical result is that only people with the resources to moderate their forums to the point that they're willing to accept liability will run moderated forums. That requires either significant resources to run the moderation program (and a chilling effect on what gets approved) or significant resources to deal with liability and American-style lawsuits. The breastfeeding mother who's setting up a blog for her three closest friends to comment on will technically be fine, but she can no longer grow her community beyond her three closest friends. The powerful (whether that's "big companies" or "socially powerful races" or whatever) are happy to keep restricting what happens on their platform and take on liability because that liability will affect them less.
I don't have a solution for this, and I find your argument mostly compelling, but I don't know that it gets us where we want to be either. I'm not really comfortable with "The price we pay for keeping MRAs from derailing forums is allowing people to say that feminists are derailing forums," but "The price we pay for making sure feminists can speak is making sure MRAs can speak" feels equally uncomfortable.
(BTW - I upvoted both of you because I think you both made good points.)