Yeah, it's read-only.
IMO that's still better than other sites, where they don't federate at all.
On Reddit, you can screencap something, and paste a static image. You can't see the comments underneath at all.
I expect instances that don't fully federate will grow slower, so it won't matter, most content will be on instances that are open to fully connecting with others.
It's up to the individual server who they federate with.
If you want a gated community, or you want to cut ties with an instance that is full of spam or unwanted content, that's still possible.
In the swarm of starships, you aren't obligated to dock with everyone.
The Fediverse is like a group of interconnected neighborhoods, each representing a different social platform or community. Just as you might have neighbors on your street who interact and share things over time, these communities can communicate and exchange ideas. These neighborhoods may differ in size or focus, but they all exist within a larger universe - just like planets orbiting around a sun. In this case, think of our star system as something like Friendica (the first step towards the modern fediverse) and Diaspora* (itself originally based on Friendica). And just as there might be traffic or communication issues between planetary systems, there could be friction between distinct Fediverse networks. But ultimately, we’re still talking about a united whole here! Finally, imagine spaceships zipping back and forth through space linking up these distinct points of interest - say hello to the protocols bridging connections between decentralized nodes known as “federation” today. It’s all connected!