"Most journalists will only rely on sources they really trust to give anonymity to. The dangers of a career ending story are too high, otherwise."

Not true. By and large there's very little risk in citing an anonymous source.

Take this 'story' for instance. An unattributed source claims that certain things were discussed in unspecified private conversations. There's zero risk in publishing that. No one can refute that something wasn't said in an unknown conversation. And even if they try, it's just a he said-she said situation.

Journalists don't destroy their careers by reporting rumours.