Why?! The whole point of federation is to let people join communities even when they don't have an account in the same server.

For people who've used lemmy or the rest of the fediverse yes, but most people don't know that yet. If someone shares a post from your site with their friends or a facebook group, they're not going to look into how lemmy works to sign up elsewhere.

1. people that are looking for a community in a niche interest, do not find it, and go back to Reddit.
2. people that are in a big instance and create (or sometimes, recreate) a community for a popular topic. This happens quite often and not because they were not satisfied with the existing communities, but just because they could not find them.

The idea of having topic-specific instances is an attempt to mitigate issue #2.

I'd prefer it if topic specific instances were more popular too. I just think that letting people making accounts tied to their favorite topics would get more people interested in joining them.

I feel a technical solution like federation pulling in lists of communities with would help more with discoverability.

Not my experience. A few examples:

• No one complained about the mods from !linux@lemmy.ml, yet I've witnessed endless discussions about moving away from lemmy.ml.

I'm not sure how that goes against what I said. That's mostly people disliking the admins.

• Beehaw defederated from LW, so this forced users of these instances to "choose" between the communities and/or create accounts on both of them if they wanted to keep following the whole conversation.

Similar issues could happen even if users are separate from the communities. Beehaw could defederate your instances, and lemmy world could defederate programming dev or something, and people would need other accounts if they want to see everything.

• Personally, I do not want to join or participate extensively in communities that are on LW if we have a topic-specific instance for it. I know that I am not the only one.

Me too. I usually avoid lemmy world communities unless there isn't an active community elsewhere.