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.
I tried mutualaidhub.org - and found another one that is about 45 miles from me so I went to their site to check it out. From what I can tell, it's nothing more than a hyper-localized version of gofundme.com. It seems most of these things are just links to facebook groups. I don't think these things are as organized or as helpful as your original post made them out to be.
Also, for the record, I'm not actually looking for assistance. I've honestly never heard of this thing until your post and just am trying to learn more about them, what they do, who and how they help, and maybe find something I could contribute. These things do not seem like a very viable alternative to traditional social services.