The issue with that is that an user could be on a popular instance, like lemmy.world or a related one like lemdroid, and search for a community on it. They could find a ghost community that was created unofficially before the self-hosted one. In that case they could think this is it and there's no real discussion to be had on Lemmy.
It is also slightly weird because there's an incentive for developers to grab the appname@popular.instance to ensure they can use the name and link it to the official instance. But that also leaves a ton of pretty much barren communities.
That's why I think keeping in sync would be a good feature, keep all communities in sync with the official one so that users aren't lost.
That said, this only works for official communities, and maybe(huge maybe) regional communities that have a self hosted instance
Doesn't it place higher the communities that are on the same instance as the user?