You could make a partial relay, but then anyone subscribed to that relay would only know about the posts, likes replies from other people also subscribed to it. But the post would still be out in the wild and able to be commented on by strangers, so there would be lots of missed reactions. 

Bluesky's thinking is a lot of people would really hate it not knowing how many likes their post got, or if there are replies that they are not seeing. And posts can get a lot of replies and likes from strangers, and from there good conversations can start. The only way to ensure everyone sees 100% of the likes and replies on a post is if there is a single relay. I kind of agree. For example your reply is not showing up on clients Primal or Iris, but it is showing on Snort and Nostrudel. I would never have seen your reply if I didn't come to Snort to check out this same post after seeing it on Primal. That is very annoying, on Nostr I always suspect I'm missing interesting replies. 
 

 Context: I've setup a different mix of popular relays on each to try and simulate the experience of a group of users. But also each client does filtering too, so that adds another "can't be sure" layer.