I've been lurking on these threads and enjoying the debate. Also shocked by the difference in how things are discussed here (robust, intelligent, fair debate) and the hysteria on IG...
One thing that strikes me about the 'say something nice or nothing at all' and 'be kind' defence is the blurry lines between personal account and business of the insta influencers.
I have a closed and tightly controlled Instagram account, but if I posted a picture of my new kitchen tiles or my new outfit and someone left a comment saying they looked rubbish I'd be a bit miffed and probably would think people should ignore/unfollow rather than make a 'mean' comment about my personal decorating choices (though arguably if I'm that sensitive or bothered I probably shouldn't post them on the internet)
However if I published a paper at work and had feedback that it wasn't well written/was inaccurate/wasn't following my industry guidelines I would never feel like the criticism was 'mean'. I wouldn't be jumping for joy but would address the feedback in a professional manner.
Equally whenever I have run events the feedback afterwards has always been anonymous. Because people tend to be more honest when they are not talking directly to you/trying to impress you.. It's not 'behind your back' vs 'to your face'. And getting feedback on something you are doing is a good thing even if that feedback is about how to improve things.
As soon as you move into a business context, whether that's your own profession or when you are buying a service, purchasing a product from a business, watching an advert, reading a published novel or article it's much more complex than 'be kind' or 'be nice'. I think it's more important to be fair and be honest.
But because these accounts either started off as personal and become business or are still a bit of both, there doesn't seem to be that dividing line between professional feedback and someone being mean to you personally.
It is possible to live your life with the motto to be kind and thoughtful and fair and still be able to give robust criticism and feedback either on a professional basis or to businesses you engage with.