Ok, now what if I were to tell you that email itself is a federated system, exactly the same as Mastodon.

All of your questions can be answered with "what does our currrent email system do?".

Sure, most people don't run their own email system. But some do. And you are not forced into a singular option that everyone is required to use.

Despite what people might claim, Hotmail, or numerous other email options are all almost as good as Gmail. Having competing options is good enough, as if you get banned from one, you can move to another (with some friction, obviously).

On expense, well same answer, what does email do? Yes, it is probably more expensive for all these different email servers to exist, but clearly that's OK, as they exist right now.

Now, seeing as tweets and social media posts don't seem like that much more of a difficult problem than email, imagine if those things were federated as well? This doesn't seem to hard to imagine happening.