This is an interesting article and one that I fundamentally disagree with... but I do love a challenge.
You're right that this is a fascinating time to be alive and public discourse and debate has changed. Gatekeepers are gone, replaced in the large part by demagogues and supporters. Although we can respond and react to injustice in real time, we're frequently bogged down with the trivia of the everyday lived experience.
Rather than talking about the emancipation of child brides in cultures, we talk about whether men move out of the way for women when they're walking down the street. Rather than discussing changes in Russian law around homsexuality and being beaten to death we're dealing with the minutiae daily interactions and microaggressions. Whilst these things are important to discuss we're in danger of not seeing the wood for the trees.
When your 'legitimacy' to comment is based on nothing more than an immutable concept, we have a problem. To fully dismantle racism, we're going to have to listen to racists. That's a given. They don't get less racist by being shut out of the conversation. And yes... whilst it might not be prudent for all white people to talk about race, or men to talk about women... in some instances, professional expertise outweighs lived experience. Being guided through difficult times needs the best brains, not the loudest voices. See Trump for an example of what happens when you pick a mouthpiece over a thought process.
Whilst 'cancel culture' continues to exist, it stands as a beacon to atavistic knee jerk reactionay thought. It's the mob mentality with digital pitchforks. For the small or large business owner to take a stand against 40,000 angry people is a huge gamble. Much easier to say goodbye to an employee and toe the line. See Milgram for why 'the bigger boy made me do it' defence is the hallmark for what happens when people lose their sense of moral accountability.
What the Harper letter will eventually be seen as, is a warning about the rise of echo-chamber activism and the dark place it will lead society. The people who signed that letter did so understanding the backlash it would likely bring. They should be listened to.
Having refuted your article as best I can... I'd now like to say how much I enjoyed reading it. I'm going to link it at the end of an article I wrote today because I'm a big fan of people getting to hear both sides of a debate. If you'd like to read my take, you can find it here. https://medium.com/lucid-nightmare/in-defence-of-the-harper-letter-a5ac3403693a
