Communities on different instances about the same topic should have the option to essentially federate so a post on one appears on all of them and opening any of them shows you the comments from all of them. This way when lemmy.world is down its not a big deal because posting to any news community federates to all of the communities instead of barely having people see your post. Federation could be decided by the community mods and the comments can have a little “/c/communityname@instance.name” on it so you know which community the comment was originally posted on.
That’s the beauty of decentralization and should be encouraged that way. Those are two different problems though. The issue of different servers with the same community topics is being figured out right now, the devs have a couple different ideas on how they’re tackling that. The other issue is onboarding, so finding a server and signing up is much simpler and streamlined. These are both issues that can be greatly improved upon.