In order to be able to choose an instance or change it in the future, a user needs to know what it is first. Maybe we could choose it for them randomly (would that be ethical?), but they still need to know where their account is hosted. If their instance gets defederaded by other instances, then they also need to know what defederation is and how it affects them. Those are just basics needed to fully use the Fediverse and the freedoms it gives you. I’m not saying that people should know exactly what ActivityPub protocol is and how it works. But they need to know the basics about how federation works. This is nothing complicated, so I don’t think I’m asking for much.
In centralized platforms users don’t have freedom, so they don’t have to worry about any of this. If they aren’t willing to learn a few basic things in order to have freedom, then there is nothing we can do for them. We can and should try to educate them, but many people don’t see a problem with being abused by corporations. This is not a new problem in software. It has existed for at least 30 years since the Free Software movement was created.
Lemmy has many UI and UX problems and they certainly need to be fixed. But it’s possible that decentralized platforms will always be more complicated to use than centralized ones. If people aren’t willing to learn how to use it, that’s a problem with them, not with this technology.