I don’t think it will ever be mainstream. It’s confusing enough for IT literate people with a basic understanding of the concept, I can’t see it ever getting popular with the wider public.
My main concern is the overlap between lemmy instances, meaning me and my mate can be looking at for example a channel named ‘news’ and see different contents because we are on different instances. So you either subscribe to all news channels on all major instances and see lots of duplicated posts, or miss out on some posts.