I do think that some kind of organisational cooperative structure would be needed even if everyone were friends...

We don't need the state to organize. Look at all the private organizations out there.

It could be a tradeoff worth making, though, if it turns out that a significant number of people are aimless and unhappy unless they have a cause to fight for...

The cause might be something created artificially by the FAI. One idea I had is a universe with "pseudodeath" which doesn't literally kill you but relocates you to another part of the universe which results in lose of connections with all people you knew. Like in Border Guards but involuntary, so that human communities have to fight with "nature" to survive.