I think the problem is that modern karate bunkai are too influenced by jujutsu and Japanese martial arts, where committed attacks are a large part of the preparatory training.
The techniques ought to be much more proactive, starting with adopting a proper posture to recieve an attack. Since a blade often comes out of nowhere, you need to stand at an angle that creates affective distance when a suspicious person closes in.
Receiving techniques are also designed to present minimum exposure of the limbs to injury by presenting the bony side to the attacker.
It's all still very chancy, but one effective defense I've worked against a rubber knife is gedan barai/ura zuki using sen no sen, against a stab for the belly (coming in at about 8:00 from the defender's vantage point). You *immediately* (attack as soon as it is presented, because you will execute during its extension) press the weapon arm and hit while moving forward. From there you can run away by moving the the right side or attempt to apply a restraint. One of the better ones looks like the awful, awful "X- block," but isn't. It's more like the FMA idea of controlling the weapon by attacking at two different angles.
This is different than defending step through thrust with a step and block. It's proactive and explosive. It *still* doesn't work all the time; luck is a big deal. Plus, the danger is that your commitment creates the problem, when you could have run away. Then again, if you run, you might get stabbed too.