That is the point, although the likes of Johnson would have you believe something else. An ambassador's communications with their government should be strictly confidential. Whoever leaked them has committed a serious crime. As you say, what was in those communications is not really new and not really controversial. I don't think there was any need for him to resign, and the British Prime Minister needed to be strong enough to say that we decide who the ambassador is, not the host country. We couldn't for instance have the Chinese government insisting that our ambassador to China should be a communist. And having decided who the ambassador is, they need to be free to report back on what they find out about the host regime, even if that regime were to disagree.