Agree with @[email protected]
Note that the one that has taken off; Mastodon - has a user face that is as easy as Twitter. Also note that something like 500k out of 4 million users are using Mastodon.Social, the main developer's instance.
I think people will naturally migrate to the most easy to use/popular option in a decentralized fediverse.
OP is right that if the decentralized tool isn't easy to use it won't gain widespread adoption. For example, WordPress is open source software that many now host themselves, but majority of WordPress users don't host their own instance, they use independent firms or they relay on WordPress.com(mercial) to set up the instance. I see that being the same with the fediverse. Actually there is a few companies in Europe already providing fediverse hosting; like https://masto.host/
Sustainability-wise; I still believe the $1 per user per year price for social media is sustainable and could be used for all of these fediverse social apps (though peertube may be more expensive with bandwidth.) Imagine Mastodon.Social, with its 500,000 users, all paid $1 a year. Eugen would have a decent amount of cash to maintain the servers and hire developers. The barrier is the global poor not having that kind of money (could be subsidized by the rich), and payment processing barriers because of US control of the financial system. As crypto becomes more widely adopted that problem may diminish.
There's also the co-operative model; Social.coop is a mastodon instance for example where the members of the social.coop site vote for the board of directors (one member one vote). Lemmy.ML for example could operate as a co-op where each member pays an annual membership fee to vote for the board and optionally major referendum/bylaw decisions (like what development should focus on, setting prices, etc.)