Well, if you can't run, ...maybe you have to protect someone else? ...or whatever, ...you have to get control of the knife, or the knife wielding arm, and momentarily stop the lethal damage it can cause. That is step one. Then other possibilities emerge.
---I agree 100%! Its just that trying to grab a sleeve is a bad idea in my opinion. As you pointed out, not everyone will have sleeves! And if do, they may rip easily. Or they may be loose. Not having positive control over the limb itself means that the knifer can still twist and turn and use his knife whether you have grabbed a sleeve or not.
The problem I have with most knife demos I've seen is that they underestimate how insanely hard step one is to accomplish. They gloss over it almost as if it were a foregone conclusion, and then go on to dazzle and distract with steps two, three, four, and beyond.
---Yep! The Wing Chun Gan/Jum is something that both Marc Denny and Mike Janich have adopted as one of their main knife defense methods. I greatly respect both of these guys and their abilities. This is defense number one against someone throwing a "sewing machine" attack, which is one of the most typical things you see in street footage. This is the guy throwing wide rapid stabs over and over. The Gan/Jum structure acts as a funnel so that whether you catch the thrust high or low it funnels to the center and you rapidly use an inward Lan Sau motion to trap and secure his arm.
---And the problem with being on the ground is that if you lose control of the knifer's arm, even for a split second, that is enough to take multiple stabs to your torso. What would have been "pitty patty" strikes that you could ignore in an empty-hand situation could easily be deadly stabs when a knife is involved. On the ground you have no ability to step back quickly or to angle out of the way nearly as well. If I am empty-hand vs. the knife, being on the ground is the LAST place I would want to be. And I've never seen a knife defense system that taught anything any different.....meaning....to take a knifer down and try to deal with him on the ground rather than doing everything you can to stay on your feet and either neutralize him or get the heck away from him! But I guess there may be some out there.