I only showed one way to grab the wrist. It was a low grab, done at the wrist joint. I recommended not holding high on the arm.
You would be welcome to hold my wrist in any way you liked and you could not prevent my cutting you with that kind of knife as you demonstrate. And I doubt really that you could prevent anyone from cutting you as you demonstrate--except, again, perhaps someone you had trained to be ineffectual.
But the primary point you're pushing is backward.
The guy with a knife
, coming to attack (and therefore needing to be stopped) is, by aikido definition, uke. The aikidoka is centered within himself (or herself) and does not go out and attack--except to "attack the attack" as Mochizuki Sensei put it.
The unarmed person being attacked or grabbed is the aikidoka.
Look at this video of an actual abduction.
That is a real example of a real wrist-grabbing attack of the kind that happens every day in the real world. Do you think the attacker grabbed the girl to stop her from using a knife she was holding? Of course not. And this video shows exactly the scenario on which aikido is based, regardless of what you imagine the samurai did.
I trained both my daughters to respond to that kind of attack when they were much younger than that child, and my son was responding to such attacks in aikido demonstrations when he was four years old. If I could have spent half an hour with the little girl in the video before that guy grabbed her, the outcome would have been entirely different. She could easily have turned his grab into yonkyo, shiho nage or gyakute seioi nage, maybe throwing him or maybe just giving her the chance to run.
But what would be the relevance of confusing the matter with relating the attacker's grab with somehow stopping her from using a knife or "drawing a sword"? That kind of abstract imagination isn't even useful for adults.