Let me ask you a question: if an instance popped up that said their purpose was to collect child porn, would you wait until you saw some child porn before defederating? What about something more mild, like if their mission was to dox and harass climate activists?
I think the exception to defederating being a last resort is when you have an instance that says upfront that they are going to be a problem. In that case, I think we should believe them.