That's not exactly how it's working in practice.
Sure, for the top 5 lemmy instances, that's kind of how it's working. But for all other lemmy instances, when you load their communities and filter by "all" instead of by "local", you are only seeing the communities that specific instance has become aware of (by virtue of that instance's members manually subscribing to foreign communities on foreign instances).
Since the very nature (by design) of lemmy is to be fragmented, it's almost a foregone conclusion that users of most instances will never even become aware of that the most popular foreign communities are for the topics they are interested in, without resorting to 3rd party search tools and community trackers/locators.
The very design of lemmy actually actively promotes fragmentation...fragmentation not just among the user base, but among communities of identical topics as well across different instances.
The only way it would be 'solved organically' as you say, is when fragmentation is minimized by just having a few super-massive instances -- but that seems to be counter to the fundamental ideals of lemmy itself.
Personally, I think this is a huge usability problem that needs some better technical solutions.