No, I disagree with nostr:npub180cvv07tjdrrgpa0j7j7tmnyl2yr6yr7l8j4s3evf6u64th6gkwsyjh6w6 that the UX would not be much different.
With Pablo's proposal, once a user fires of the kind:1777 to initiate migration, if your client supports it, depending on how the client implements it, you would not be seeing the attackers notes, or they would have a red border and warning signs telling you that these notes were written after the valid kind:1777.
Today the client has no way to know. With kind:1777 your client can now warn you about the situation.
Of course it's not a mandatory feature. But it's very central and I think any client that relies on kind:3 or other lists will want to implement it.
The 90 days was an idea to prevent front-running by a few clients, as we have seen in the past, it's how long the fast implementers wait as a curtesey to the others, not a deadline.