In such a widespread usage, there would probably not be "the fediverse" anymore, but a galaxy of clusters of interconnected instances. Spam would be a serious problem, so instances would switch to whitelisting instances they want to federate with instead of just occasionally "defederating" from them. It would not only happen because of spam, by the way, but also because of political/cultural/ideological divergences. Maybe even because of laws.
There would be a boom of innovations, made possible because of the data openly accessible and the fact that we would finally have a standard on which to build upon to create third party applications (which, from a developer perspective, was the promise of the web-2.0 and its APIs, but never truly materialized). You would see alternative frontends for everything, and applications that allow to get new insights or use your data in new and smart ways.
The big businesses would still be around, by the way. They would open their own instances, publish lot of ads and add cool features found nowhere else so that most people join their instances, which would quickly become the go to instances for everyone, dwarving all other instances. We would spend a lot of time evangelizing so that people join smaller instances instead, but our folks would answer that it's less convenient, they would have less easy to use features and their account is already at BigCo anyway. Plus, to fight spam, terrorism, child pornography, nazis or whatever is the scarecrow then, they would severely limit the possibility for small instances to interop with them, adding arbitrary technical barriers that most implementers won't succeed in hoping. But we won't care that much, because we will have our own alternative networks with more content on them than ever.