I think the way federation currently works spells doom for the fediverse, should any service of it get major traction. Currently, if you subscribe to a community on Lemmy or follow a user on Mastodon, your instance will pull the content of that instance/user and make it available for all to see and interact with. What seems like a good idea to spread content however is becomming the achilles heel of the fediverse: The admins of Lemmy/Mastodon instances are liable in many juristictions for the content their servers are distributing. This means in practice that many Lemmy/Mastodon instances block NSFW content for example, as the admins, understandably so, are either unwilling or incapable of making sure they are not running afoul of any laws.
As such, I think that the fediverse needs to offer a way for users to follow content from other instances without having that content be stored, let alone shared by their home instances.
A question I have at this point is where this criticism is best levied against. Is it the job of Lemmy/Mastodon to provide such a form of federation, or does the ActivityPub protocol needs to be ammended?
That is not a proper solution. Especially when Fediverse applications are becoming mainstream, most users won't switch instances to access content they currently are unable to. It will just lead to endless support tickets and -threads on why they can't find what they are looking for. Also, for people that rely on exposure, such a fracturing of the Fediverse is just not worth it. Furthermore, having to restrict access to content in order to cover yourself legally leads to self-imposed censorship worse than even big platforms, like reddit, currently demand.
You bring up legitimate concerns, but I think the bigger point here is that the Fediverse is not Reddit, nor should it be.
I find that fragmentation mitigates the system manipulation and "gaming" that is often tied to people who rely on exposure.