To illustrate op's point I'm going to spin up an instance, federate with everyone, and not tell anyone what that instance is.

Then I'm going to feed all that data into my new website, called Open Lemmy Stats, where anyone can query the user data ive accumulated. The homepage will be ripe with insights, leaderboards and all kinds of data on prolific users.

Additionally, I'll display a snapshot/profile of a random user by feeding that users data to GPT4 to make inferences about the user's political affiliations and display the results.

Worst of all, I'm not going to out my instance for everyone to know it as the one to defederate. In fact I'm spinning up a few instances that will host innocuous communities that I plan to mod and support to give my instances cover for their true purpose: redundant fediverse datastreams for my site, Open Lemmy Stats.

I'll also have a store where anyone can buy my collected fediverse data for a handsome sum.

Just kidding I'm not doing any of this. But someone absolutely will or already is.