The rest we'll have to disagree on. I think they are important observations and also indicate some of why this is happening now.
Sure, I think it's good historical context for broad understanding, but I don't see it as effectively rebutting anything in the letter.
In addition, the "cancellation" of minorities has also been unaccountable to anyone, either. Who could be the correct source for accountability? I don't think there is one. In democracy, we turn to "the people" for accountability, so a "mob" is an accurate representation of our society insomuch as there can be an accurate representation of people who are kept from power via disenfranchisement. American government is not holding bigots accountable, and even those trying to go through the current channels to fix that issue are being blockaded.
Beyond the first sentence, I can't agree with this. Democratization is happening due to social media, which is good in some respects, but has a lot of negative consequences. The big three American social media platforms (Twitter, Facebook, Instagram) have designed themselves in ways that either promote disinformation (Facebook) or incentivize users into nasty dogpiling, context-flattening, and give disproportionate and overrepresentative voice to extreme viewpoints (Youtube does this too). Given that this is the medium through which mobs operate in 2020, there is no think these platforms are going to be any sort of fair or representative mechanism for justice.
Put another way, I don't want twitter/facebook mobs running the country any more than I want them running who publishes at the Times. We effectively have a Facebook President right now. It's not going so well. You can think we're facing widespread institutional rot (because we are) but that doesn't mean you replace the institutions with an app with a bird logo.
In addition, there is no attack on freedom of speech. People can and will say what they want. I find that premise to be completely laughable and that's part of why I find this letter so pretentious. Yes, the public, especially on Twitter, is holding people accountable for what they say, but that is not an infringement of free speech. To use a very obviously example, if I were to call someone the n-word, I would be shunned by people of good conscience, because I would have displayed my thoughts on Black folks. What is happening beyond that that requires such an outcry?
What I am concerned about is
disproportionate punishment for transgressions. Online mobs premised around public shaming are purposefully vindictive and seek to produce lifetime consequences for people with no standard for redemption or rehabilitation. You don't need to think "free speech" is under attack to be inherently distrusting of a culture that promotes these norms. I find the cancellation movement abhorrent because I value fair and restorative justice, not because I'm especially interested in the opinions of transphobes.
Speaking of which...
If people like JK Rowling are upset that people don't want to put up with their bigotry, okay, fine. But don't blame a lack of free speech or claim you're being silenced or that it's an "illiberal culture." It's people being tired of dealing with bigotry and explaining why it's bad and cutting you out of their lives and letting others know that you want to keep spouting bigotry. This pandemic has us all exhausted, and knowing that someone likes to say awful stuff means I can avoid having to deal with them and save myself some energy.
I'm not especially interested in Frums, Gladwells, Weisses, etc. of the list. The vast majority of signatories are people who have committed no easily-identifiable transgressions. A letter that was signed only by J.K. Rowling would be uninteresting. A letter that is signed by writers, researchers, journalists
who a healthy liberal society relies on saying they feel unable to perform their work in the current climate is what makes this worth taking seriously.