The problem with this is that rollbacks would set a worrying precedent. Could any Tom Dick or Harry complain of fraud and demand a rollback? Could a nation State demand a rollback.
Then doing a rollback now will not change anything. The possibility has always been there, and always will.
And let's take a look at your scenario:
If Tom, Dick, Harry, or the Queen of England demand a rollback, then it will simply be up to the NXT community to decide whether they want to jump onto the new blockchain or keep using the old one. Just like today. And today, we can demonstrate initiative and cohesion around the cause of protecting our currency and fraudulent use of NXT, without even touching any other transactions in time.