We had a P2P and open source Reddit alternative 40 years ago, called Usenet. But people fell for a centralized one run by a company once it came along, like they always do. May Reddit's fate end up looking something like Digg's and AOL's.
But a minor quibble: USENET wasn't exactly peer-to-peer. I mostly got to USENET through my university or ISP's NNTP servers. If I wanted to start a new group and get you involved, I would've had to set up my own NNTP server for you to connect to, using either your NNTP client or your own NNTP server.
A lot of people seem to believe that centralization is the problem, and ignore Wikipedia, which has been true to its mission far longer than Reddit or Twitter have, and it never needed a decentralized model. The simple explanation is that it's the profit motive that is the problem. That can be fixed with the proper organizational structure.
I wish the Mozilla Foundation would stand up Reddit and Twitter clones. I bet they would be more usable on Day 1 than the Fediverse solutions.
Well-managed centralization does allow for governance, where things like moderation and technology decisions are difficult to leave up users. It's sad what happened to the public square of Twitter. There really isn't a viable alternative at the moment which has the reputation of a neutral ground where people can exchange discourse with relatively even chance of being heard, for all its problems.
Wikipedia's original vision was 1) to free the content and the communities for forking as well as copying, and 2) to give everyone in the world a free encyclopedia in his or her own language. I have a big rant about that but the quick version is that Wikipedia.org (the web site, not the project) should have been designed like Github, so you would use it for editing and creating forks, but (mostly) not for reading articles. Instead, whenever you buy a new consumer PC, through deals set up by the WMF, Wikipedia in every language should already be on its hard drive (you can delete it if you want the space instead). You'd read articles locally instead of through a privacy invading web site and internet surveillance apparatus, you'd get updates through a nightly equivalent of git pull that got all the changes with no indication of what you had been reading, you could downstream your copy to your friends or followers, you could make a fork and downstream that if you don't like Wikipedia's policies, etc.
I agree with you that Wikipedia doesn't have the exact same problems as Reddit. It is arguably not as bad as the current version of Reddit (we're here right now because Reddit is about to get much worse), but it has its own problems, spreads propaganda that is taken more seriously than Reddit posts are, etc.
Come to think of it, we also had a perfectly good distributed, peer to peer, FOSS source control system called Git, but it got mostly supplanted by a centralized "hub" now run by Microsoft, the original closed source empire. It never stops.
And to get there first: please don't bother saying that forking Wikipedia is permitted under its licensing, since that is near irrelevant in practice. It could and should be made simple and easy. Wikipedia's main value these days seems to be AI training rather than informing humans, which was supposed to be the point of an encyclopedia.
Coming back to your first point... how often would you maintain a fork of your own encyclopedia? Even with Git, you would fork mostly in order to submit a PR to the original repo. I'm just trying to understand the use-case here since I cannot imagine a situation where I would fork Wikipedia and make my edits to be shared with my friends only and my friends would do the same.
Did you mean something like fandom.com where communities (not individuals) can start their own wikis? I'd also love that.
As a Wikipedia example, look up Joy Milne (the lady who can smell Parkinsn's disease) with a web search, then look her up on Wikipedia. Wikipedia says nothing or almost nothing. That's not an oversight, that's Wikipedia bureaucracy in action. You might be able to get an edit through by battling the bureaucracy for long enough, but if you have an interest in this illness and wanted to gather and share info about it, it would be simpler to just fork the PD article. Et cetera.