I think it's true they won't offset the large platforms, but I also don't think those communities are rare. I too use Discord and YouTube (begrudgingly), but I also use IRC, wikis, and old school forums. There's also Matrix, mastodon, lemmy, mailing lists, heck even usenet. But what's rare probably stems from us being in different communities?
My opinion is that the communities I've found the most fulfillment/happiness in have are not funded primarily as a commercial operation. Sometimes they are related to commercial things (fandoms, user groups), but the community itself is not trying to sell itself, its users, its data, etc. My opinion is that it would be better for all internet-based communities if there were lower-friction ways to start an online community without giving yourself up to commercialization of a platform. Through the many layers of tech that would require changing/tweaking to fix this. All the way down from network layer DDoS mitigation to application layer maintenance and hosting.