There’s also the stuff that “you get to know before it becomes news”, which also won’t be on Mastodon because it lacks the “gossipers” and the mass of users that is needed for having people “everywhere”

It's not the gossipers, it's a trending view. Mastodon doesn't tell users about active conversations as they happen. That makes it really hard to get breaking news, because you need to be following someone who is posting about it AND you need to recognize that it's breaking news.

Maybe that has improved in the year or two since I used Mastodon.