To be fair, that second model is what bluesky has. You can run another indexer, but unlike mastadon where you only have to deal with interactions to posts that

To be fair, that second model is what bluesky has. You can run another indexer, but unlike mastadon where you only have to deal with interactions to posts that involve your instance's members, with bluesky you have to be able to keep up with the entire firehose at once. So its cost prohibitive for all but big orgs. It does mean search (and as importantly, likes and comments) are globally consistent so there's def a trade off.