Thanks @aschrijver , this is very helpful.
I take from the first paragraph the idea to “use it wisely”.
About the second paragraph, I was referring more to the idea of what it enables/does, than the protocol itself, when talking about “ActivityPub”. The “sharing/linking content” idea, or “freeing the content from servers boundaries”, like you talk about it.
The idea of some kind of shift of control/power is interesting: It would not really be the community owners who control anymore “their” users, “their” content (what they host, at least), what people have access to when coming to their place, how information is organized and grouped, etc. The users would be more in control and more free to pick what they want, from where they want, and make their “own menu”.
I can see how this may be appealing from the user’s POV, and how it may be a little scary/worrying from a community owner POV.
Taking a restaurant analogy, I agree I was probably getting a little too far with my point of merging several places, but I think your analogies are too soft: It’s more than what you describe, IMHO. It’s going to one restaurant and being able to order a dish from another place, made by the chef of that place. It may raise questions (which was a big part of my point) as to why the restaurant owner who pays well that chef, and maybe had troubles to attract him and retain him, would want to NOT have a clear reason for customers to come to HIS restaurant anymore. Your answer is kind of: It’s great from a customer POV. Yes, sure, I agree.
But anyway, you may be right on this, and the point I’m making looks quite like the fears of companies with open source in the past.