So activity pub is the common language of a federated system sort of like smtp is the common language of a federated system. or html is the common language of a federated system(html is strange however as it's transport model is inverted compared to the others, it moves the user to the message rather than the message to the user)
The reason federated systems work is the same reason capitalism works, and not incidentally the reason federated systems don't work is the reason capitalism does not work. my conclusion is that capitalism is a federated system of doing business.
I suspect the main reason federated systems work as well as they do is that there is individual ownership over the pieces, each person caring for their own piece. Some will win some will lose. It is not nearly as efficient as a centrally controlled system, but is far better at dynamically adapting to changes.
On the whole I think federated systems end up bringing the greatest good to the most people, but they can go very wrong, mainly this happens when one player gets too big and stops playing by the rules.
On the subject of reddit specifically. on one layer reddit is just a wep page, it provides the forum discussion pattern to people. The web is a federated system, there are many forum discussion patterns available on the web. when reddit fails to provide your forum needs you move somewhere else.
On another layer, reddits success was in no small part in that it provided a sort of ownership over each forum, and each owner was responsible for their own forums success, and when reddit starts infringing on this ownership it feels like something is being stolen from you.(At least I think this is what is going on, I have to admit I am only an incidental user of reddit and the whole situation confuses me) Where if the system were more federated from the start it would be more like actual ownership and less like feudalism.