I am with you as a user, but also an instance administrator. Forcing our hosted communities together with federated communities would take away nearly all motivation I have to host an instance in the first place.

Then just write in a .txt file on your computer using notepad.
You have to understand, the point of social media is to come together.
It is very easy to fragment into ever smaller group, it will NEVER be difficult to be excluded.
It simply IS NOT the problem we are discussing here.
The problem IS the fragmentation that is unavoidable when we try to decentralize.

Without this Lemmy becomes Reddit with extra steps, it creates the "one big community" on the "one big server" it put all the power in the hand of whoever has the key to that instance, and just like that we're back on reddit.

We have to be able to have a "books" lemmy community that exists accross the whole lemmy very, I want there to be 1500 books community on 1500 servers. I want anyone to be able to post on any of them and be just as likely to be seen.

Because if you don't then, every topic on the lemmyverse will look like this

Books@lemmy.ml - 12.7K subscribers
Books@lemmy.world - 6.56K subscribers
Books@lemmygrad.ml - 464 subscribers
Books@sh.itjust.works - 233 subscribers

And hundreds more with less than 100 subscribers, where posting could not be seen by even 1% of 1% of 1% of users ?

This puts all the power into the Books@lemmy.ml mods and the lemmy.ml instance owner.

And worse, as these mods become more and more lazy or corrupt or just stop caring. That one big community fragments based on what becomes excluded from the "one big community"

So you end up with the second community getting filled up with toxic anti-vaxx and flat earthers, which further empowers the "one big community" because now the alternatives are total poison, the VERY IDEA of leaving becomes unthinkable.

This is the logic we are fleeing Reddit and Twitter from, this is the logic that created the horrible places like Rumble, Gab, Parler and ducking "Truth" , which become empowered by in their toxicity by the centralisation and polarization of the "one big community"

What I'm saying is that you're basically making an "all lives matters" argument, yes it's true but that's just not the problem, you can make private, invite only or communities with incomprehensible and unassociable names. Nothing is stopping your leaving in the lemmy woods and never being seen again.

That is just not the problem at hand.

Then just don't make the post a crosspost ?

If you don't want what your posts being seen, put them in increasingly obscure places, I'm pretty sure we can even have invite only communities, or undiscoverable communities that can stave off eternal september forever.

But this is a social network, it exists for pro-social reasons, it should not cripple itself for the sake of the unsocial. Without agglomeration systems, lemmy is becoming the same as reddit, and that will eventually lead to tyrannical owners and mods, community fragmentation where everyone becomes a mutual hostage-taker of everyone else to the benefit of moderators.

If you don't want to become part of the community, there are many unfederated servers you can use which won't even be visible by outsiders, from your description I think that's what you problem is, you don't want to be on a federated server. At least maybe one that doesn't allow outsiders in.