I have also thought this is a good idea. I think that the ActivityPub standard should have a required field that lists a copyright license. Then a copyleft style copyright should be created that allows storing and indexing for distribution via open-source standards, and disallows using for AI training and data scraping. If every single post has a copyleft license then it would be risky for bigtech to repurpose it because if a whistleblower called them out that could be a huge class action suit.
A good question is if a single post can be copyrighted. I think it could. Perhaps you would consider each post like a collaborative work of art. People keep adding to it, and at the end of the day the whole chain could function as a “work”. Especially since there is a lot of useful value and knowledge in some post threads.
ActivityPub is all about pushing content around to subscribing servers. It sort of expects the subscribers to always be online which would not work for a phone. Servers could resend missed events, but essentially you would miss every event that occurs while the phone is asleep or doesn’t have the app running.
Also, every event that occurs needs to be processed and stored whether or not you are actively looking at it so it would be a huge battery drain while it was running.
It is definitely a service best run on an always-on server with a client application in a phone just asking the server for the latest stuff on-demand.