My user experience with unmodifiable and undeletable notes makes nostr close to unusable. When I post a note with mistakes that make it illegible, i'm faced with either replying to my own note with corrections, or posting a new note. This makes the recipient's experience degraded, and forces both parties into an idealology they have no connection to. The object to nostr isnt to make it some kind of pure programer's panacea, but to enable uncensorable communication while balancing ease of implementation to encourage adoption with enabling expansion of scope within the new communication protocol's M:N paradigm.
Regarding lack of consistency of relays and potential forking of versions, this is something that exisis already with any replaceable events such as the profile information, but for notes it can easily be resolved on the client side by comparing revision data.