I believe the problem is server load. You want each server to listen and send updates to the federated feed, and forward the federated feed to the users of that instance.
What you want to avoid is a celebrity on instance A posting and then users all over the fediverse pinging instance A to get that post. Instead, instance A sends the post to the fediverse and then for any given user to view the post, they ping their own instance. That allows server requirements to be more closely tied with the number of accounts on it and how heavily they use it rather than the popularity of any given account. Additionally, it makes it so servers only have to interact with their own users and other federated servers.
Of course the downside to this is that as the fediverse as a whole grows, the volume of the federated firehose increases, which still increases the minimum requirements (processing power, bandwidth, and storage alike).
There may be middle grounds here. For example, the instance I'm on (I have no idea how common this is) stores text posts from other servers but images from are not saved, it's the originating server that serves those. Additionally, there may be the possibility of deleting old posts from the fediverse that don't have any local replies, or something like that. There are issues with the latter, especially with old friends-only posts becoming impossible to access by looking at the public profile on the original server, but I could see that being solved by something like generating a readonly token for the other server or something like that.
What you want to avoid is a celebrity on instance A posting and then users all over the fediverse pinging instance A to get that post. Instead, instance A sends the post to the fediverse and then for any given user to view the post, they ping their own instance. That allows server requirements to be more closely tied with the number of accounts on it and how heavily they use it rather than the popularity of any given account. Additionally, it makes it so servers only have to interact with their own users and other federated servers.
Of course the downside to this is that as the fediverse as a whole grows, the volume of the federated firehose increases, which still increases the minimum requirements (processing power, bandwidth, and storage alike).
There may be middle grounds here. For example, the instance I'm on (I have no idea how common this is) stores text posts from other servers but images from are not saved, it's the originating server that serves those. Additionally, there may be the possibility of deleting old posts from the fediverse that don't have any local replies, or something like that. There are issues with the latter, especially with old friends-only posts becoming impossible to access by looking at the public profile on the original server, but I could see that being solved by something like generating a readonly token for the other server or something like that.