I don't believe that accessible community-building platforms are a problem. Nor do I believe it's something that can be solved; people will understandably congregate on platforms that are easy to use and that their friends are on.
Is the only healthy Internet one where there's 500 different platforms and to try and build a community you have to be an IT worker? How is it a bad thing that someone can hop on Discord or - indeed, as much as you might hate to admit it - Facebook and effortlessly create a group for their friends or local club?
Centralization is not the problem - it's what those companies can do with their power and data that is the underlying problem. And attempting to de-centralize things won't solve that problem - either some platforms will grow to a point where abuse happens, and/or the point of abuse will lower over time to cover those smaller platforms.
Without, for example, data privacy laws, nothing will change, whether we're on 5 platforms or 500.
(As a brief aside, since you brought up IRC in TFA... is it at all relevant that, even then, there were like 5 main networks?)
Reddit definitely feels homogenous to me. Yes I know, smaller subreddits etc etc, but it’s built on the same engagement infrastructure that is the basis for so much of the modern internet.
Anyone who has run a donation based service will tell you whatever % of users you think contributes back divide that by 1,000 or 10,000 to get the actual number.
The elephant in the room here is how you ethically get people to onboard without an existing community / fomo / money.
The trick is getting the content creators there, but most of them are ultimately and fairly interested in making money, and your new platform wont have that for them.
Bluesky has done alright, but that was a black swan event Elon Musk inspired.
Do what reddit did, use multiple accounts as founders, and with AI I'm sure it's even easier to do so. For content-based platforms you must have content, there is no way around it and I don't see adding fake content in the beginning to be unethical, it is a solution to the cold start problem (also a good book by Andrew Chen at a16z [0]).
It doesn't have to though, we could train AIs that push back or even coordinate with a human therapist similar to how self checkout lines still have an attendant.
Even if you go on like Reddit if you post certain things in certain subs you'll get invited to private subs talking about whatever niche topic.
Same with Discord, lot of small communities there.