I think as long as it's clear when mute is being used, people can make up their own minds about the validity of it. I may want to follow a particular user who mutes people who are disruptive or try to derail discussions, while on the other hand, I want to be sure not to follow people who use mute in an aggressive way. You're right, the determination there is subjective, so whatever mute interface is implemented, it should be clear how it's being used. I'm not sure what exactly should change, though.

As for scaling, I agree with you, something should be changed. If I want to see discussion about a particular news story, I don't want to have to search through five or more different discussions because some gun rights activist muted a gun control advocate, who wanted to respond to pro-life commenter who muted a pro-choice person, in a post by an atheist who muted a very reasonable non-denominational Christian, who were all muted from the original discussion by a feminist. It's fine if they all want to have separate discussions, but as someone who likes being exposed to diverse ideologies, I would at least like to easily find and read them all.