Questions about gossipsub metadata-only peering and fan-out

Hi there, I read the doc about pub/sub and gossipsub at and have the following questions:

  1. It says the subscribe and unsubscribe messages go hand-in-hand with graft and prune messages, and when a peer unsubscribes from a topic it will prune its connection with peers still subscribed to the topic back to a metadata-only peering. My question is – why do we keep metadata-only peerings even with peers outside of the topic instead of just cutting the connection (or, I guess, “downgrade” it to a non-pubsub connection)? Since in my understanding the metadata only peering was to assist with message delivery through the IHAVE and IWANT messages, peers outside of the topic wouldn’t be useful there?
  2. It says that when a peer gets an IWANT message, it’ll send those messages to the requesting peer. My question is – in the doc it didn’t mention that any “grafting” happens before sending the full message to the requesting peer, was that an oversight OR the full-message is actually sent through a metadata-only connection in this case?
  3. What’s the motivation/use case behind fan-out? If we allow any peers to send messages to any topics, wouldn’t that make flooding topics with non-sense messages much easier for bad actors? and how would one generally control what peers can subscribe to what topic?