Nice post! I think #3 is depending on certain factors. The reason I say that is because i, as a social media marketer, I want active engagement on my posts and on my client's posts. So disabling the comments removes a huge part of what social media is all about. Comments allow for open discussion and engagement.

Of course, there are going to be people that don't want to see those comments. Especially when they're irrelevant to the post, just as you said. But from my perspective, a digital marketer, they're absolutely necessary.

Just my opinion :)