I feel like this misses what's actually going on. The "small, sharp, ideologically extreme" discussions aren't going away, they're just happening elsewhere. From the abstract, the reason for the decline is: "the youngest and oldest Americans increasingly abstaining from social media". The young people are talking in private Discord groups, and the old people are talking in private text groups. These private groups don't show up in social media studies. The paper even states this directly: "everyday communication increasingly migrates from large, open networks to semi-private spaces such as group chats and messaging apps".