I have seen full speed practice 'work' where the defender grabs the knife wielder's forearm with both hands and then works from that position.
But against a knowledgeable attacker one must be extremely vigilant against the attacker just switching the knife to the other hand.
And a strong attacker will start flailing that arm around unless you are able to control it and advance the position -- just hanging one works for a limited time at best and psychologically once you have that "hold" it is so tempting to just hang on.
And you are correct: The issue with the BJJ stuff is both the knife and multiple attackers.
Almost by definition "your mugger" (who picks you) will KNOW in his heart that he not only will beat you but will do so WITHOUT injury. While a mugger can be wrong, they MUST be correct a very high percentage of the time or they will end up dead, in jail, or injurred -- at best they go home without their drugs, dinner, etc if they don't "win".
So if "your mugger" is easy to beat then chances are he is about to produce a weapon or his friends are about to land on your back.
He arranges the odds to be vastly in his favor and no matter how armed or how skillful we are we must prepare for those overwhelming odds.
Keep fighting -- God will tell you when you are dead.