And then again it's not very clear what we're getting from this.

For example, in the "update" event approach the same problems of contact lists remains: one client can overwrite an update event from another client and people lose part of their contact lists.

In the case of "delta" events then you must ensure that you have the full history, which means you must know the exact relays to where a person is publishing their deltas -- but if you are diligent enough to know that and you have successfully written more complex software able to handle that, then why can't you do the same for replaceable events today and fetch the damn last-updated contact list from a relay that you know will always have the last version before replacing it?

I think your suggestion of having replaceable events + delta events (I don't remember the details) could have been a better approach actually, as it would preserve the best aspects of all worlds, but I'm not sure about the implementation complexity of it.