I would say that a great percentage of people who threaten with knives or even guns are relying completely on fear and the apparent sense of power it gives them over an unarmed opponent.
This could be true, I don't know the stats but my point being that all things being equal
(which we can only assume since this is hypothetical), the knife will win out. It's rock paper scissors at this point.
These type of questions can never be answered though. All situations are different, all attacks are different, all attackers are different. Is the sun in your eyes? Were you sneezing at the moment of attack? Is there a slippery surface? Is the guy on PCP? Were you on PCP?
I don't think the person being attacked has to disarm the attacker but he has to control the knife or render it useless somehow.
The original question wondered if Aikido is an effective art for disarming an opponent with a knife. I still say no. Running is the best defense for a knife.