Less violence, on the other hand, is a clearer goal that aligns with existing laws against violence, so it has a firmer basis, in line with existing restrictions on speech. It also gives you a non-partisan measuring stick.
Private platforms, on the other hand, will have more flexibility in trying to maintain, say, discussion quality as a goal (see HN's stated moderation goals vs Reddit's). Or disinformation, or language/profanity, or whatever else a private moderator may choose.
The government telling those private parties that they can't set their own standards, though, seems like a particularly terrible direction to go.
Totally agree with your last two points.
Nuance gets crowded out because posts and comments presenting polarized opinions are much more likely to get acted on by readers than their more nuanced counterparts are. Posting and voting becomes more about the dopamine hit from the gained karma and sense of being correct than it is about discussion.
This is why while it still isn't perfect, one can often see higher quality discussions in smaller niche subreddits where there's no critical mass to push dominant opinions. In those communities, posting extreme comments just makes one look like an attention seeking troll.
So in my view, moderation or lack thereof is almost orthogonal to an online community's propensity for quality discourse. That seems more related to the designs of and incentives given by the software these communities run on.
I didn't intend to prove "bans cause homogenization", but rather cast doubt on "bans achieve the stated goal".
The goal of the platform is to pull in more users. Having less abusive language and users is a means to that end, as is appeasing general public outcries for more moderation.
The goal of the public and lawmakers is, hopefully, more effective and meaningful discourse.
The paper you link measure for the prior outcome, not the latter.
Edit: Just to comment on the outcome you are referring to though, I don't believe the bans actually lead to more meaningful and effective discourse on Reddit. I don't even thing it lead to more civil discourse. You can click on literally any politically sensitive topic on the front page and you'll find that almost all the top comments both lean in a specific direction (diversity of discourse is terrible) and the deride anyone who disagrees. Maybe they use the 'F' word less, and no one is using the 'N' word anymore, but it is very obvious that it has become a strongly homogenized platform that doesn't welcome nuanced opinions.