Nobody outside tech cares about decentralization or federation.
At some point, everything converges to centralization.
No amount of Mastodon servers or any fediverse self hosted instances spun up will change that.
There is a reason that mastodon.social is the biggest instance and that they couldn't close registrations to promote decentralization.
Hell, I would even say that threads is the biggest mastodon instance.
The very second most people need to think about implementation details, like what instance is connected to what instance and what that means, they're done. It's not like they couldn't understand it if they tried: they don't want to try because they don't care. In fact, they don't want to care.
What they care about is interacting with their friends and family and celebrity personas that amuse them, and also get some news handed to them that fits their world view. They don't want to have a little independent social network-- they want to connect with whoever they want to connect with and access whoever they want to access and any barrier to that makes it worse for them. The centralization is the selling point for a lot of those people. Being at the terminal point in a federated service, as they exist, makes all of that harder. It makes it harder for great reasons but they're reasons that most people don't even want to care about. They trade good, easy experiences with the things that are important to them for things they never wanted and have no interest in learning about. Nerds get a kick out of being on a decentralized social network. Most people absolutely do not care. It's not like they don't want to be: it just doesn't matter to them, so if they have to give literally anything up to achieve that, it's too high of a cost.
It's almost like trying to get people to switch from mobile phones to amateur radio.