> It basically means politicians can't have any genuine conversation with constituents on social media because it'll be drowning in trolls. Which I guess is fine, but it seems less than the ideal outcome.

The politician should mirror the post on a website the politician controls with no comments allowed or simply post on such a website in the first place. The politician can then post a link to the relevant page of the politician's website to the politician's social media accounts and proceed to ignore the comments on the social media site. If the politician is a government official, this is still perfectly compatible with the First Amendment, because the official has the First Amendment freedom of association and therefore can refuse to host comments on the politician's announcements on the politician's own websites.