What is "everyone"? All relays in existence? If 2000 relays exist you'll send to all of them? Why do you assume these relays will all be willing to accept your notes? If they do accept notes from everybody they will certainly blow up very easily and stop working, therefore they cannot. And if they cannot then what is the point? The outbox model fixes this.
Discussion
yes, i'm quite aware of this
kind 0, 1, and several others are intended for broadcast, and that is fine, people can decide whether they want to cache them or not, but they are ostensibly public
so they can be broadcast, regardless
i've done work building gossip networks and for this reason i wrote a flatbuffers style codec that lets the gossip node quickly reject messages that are irrelevant to it, this is really the main thing that flatbuffers style binary codecs are useful for, and counter mode encryption can be useful for this also as it lets you selectively DECRYPT the message as well and extract fields by their offfset and determine whether to decode the whole message without doing a whole decryption AND decoding