This is a beautifully written introduction to the architecture of AT, but after much consideration I will still remain on ActivityPub for the time being.
I love the idea to define data formats first, and then build on top of that. It's the only way we should do everything, because if you have the data, everything can be re-built on top. Unfortunately the way AT works is all contained in here:
> Social aggregation features like notifications, feeds, and search are non-negotiable in modern social products. [...] Coincidentally, that’s the exact mechanism you would use for aggregation. You listen to events from all of your app users’ repositories, write them to a local database, and query that database as much as you like with zero extra latency. [...] This might remind you of how Google Reader crawls RSS (rip).
In order for the social aspect to work, all data must at some point or another be aggregated in a single place. Said single place must then be huge, as it scales linearly with the activity of the network; in a still-capitalist world this means that this single place will always be run and led by money, unless some extraordinary volunteers-based project like Wikipedia springs up. The example of Google Reader is to the point: it was the biggest tech company at the time, provided a service for free, and decided to stop because it didn't care anymore.
In fact Google Reader is a very good comparison. AT works exactly as if you had websites, each with their own RSS feed, and then a big relay called Google, providing search, feeds, notifications, ... but as we all know by being the middleman between producers and readers Google gained an astonishingly high power. That is the business model described by Cory Doctorow when he talks about enshittification. Put yourself in the middle, and everyone will depend on you.
The only way an AT based product works at scale, ie with everyone easily talking to everyone, is with one or a few mega intermediaries between everyone of us. I fear this is not going to solve any of the issues we have.
What is different in ActivityPub ? Intermediaries are definitely useful for some services, but once your network is built you don't need them anymore: content flows directly between the repository, no middlemen needed.
In short: if we want a single network at large scale, AT requires large scale centralization points, while AP certainly needs them but could survive without them. Either we face that, or we start exploring and living within small-scale networks