When spam means you're either flooded with garbage or are forced to just totally block potentially innocent senders (like people running private mail servers)...
...and probably 95%+ of people I know use Gmail for their personal accounts...
...I'm not sure e-mail is the best example of federation working great.
I will gladly take it over closed protocol, even if it has reaction gifs built in.
Email is an example of a federated system failure.
30 years ago, AOL may have been most popular email provider, it was displaced by Hotmail, then Yahoo, now GMail. Having different leaders over time is evidence of the resounding success of email federation, those domains still exist and can still send/receive emails universally. If email had hypothetically been non-federated and proprietary to AOL, it would be dead today (or as close to it as gopher)
Counterpoint: email has been working great for the past 30 years.
The reason why email works over centralized schemes is because it has tiered decentralized controls - sending server, relays (when they were a thing), receiving servers and the client each have an opportunity to accept, reject or drop a message.
Mastodon has a great chance of succeeding. All it needs is an easy-to-use client,decent discovery and giving users control of what they see. It does not have to be a constellation of mini-Twitters.