Single sign in to the fediverse seems awful. Then we are logging in to it through American big tech servers. Forget anonymity and no tracking. Probably see ads on the login screen too.

But otherwise, sure. A single server is not always a bad idea. In practice, this is how Lemmy is too. Most people are on Lemmy.world, and they picked that server because they don't think decentralized is important.

Sorry but I fail to understand the relation between your question and the additional text.

Are the bulletpoints requirements for a less over-engineered attempt ?

Or are they examples of the current situation?

And just software without protocol seems. .... oversimplification.

@fediverse

All of the shared/single sign on and easy cross posting would probably be trust or allow-list based.

As the platforms would be FOSS, anyone could run their own instance and start their own "circles of trust". So even with big vs small server friction, there could be a few "gardens" of small and big server networks providing different "spaces" for different purposes ... all without having to worry about defederation and the software difficulties of building against the protocol.

Maybe good old blogosphere with comments pingbacks and pubsubhub(?) was a sort of simpler proto version of the fediverse.