As for "Crypto-less federated systems like Mastodon and friends" - you have to ask yourself why they don't work at scale. That's the key insight of Bitcoin. You need to have a system-native incentive for the nodes to do their thing, and this system-native thing (BTC the currency, in Bitcoin's case), should have outside value. Many wiser people have said this (not on HackerNews maybe), that money is the only use case for a blockchain.
I’m not sure that Bitcoin offers any special insight here.
We use email; it’s very much a crypto-less federated system that works at scale (of, more or less, every Internet user!)
And there are newer examples: while it’s by no means a Twitter, Mastodon (and the broader ActivityPub universe) is already at a healthy scale. I collected these stats three and a half years ago; the ecosystem has grown substantially since then: https://davepeck.org/2018/05/03/mastodon-stats/
