To sum up my take on some of this thread:
Aikido is a wonderful art with many benefits and principles of movement, some of which can aid in actual SD, though many techniques are difficult to execute against resisting opponents, and only then by expert practitioners.
Unarmed defense against a casual or impaired knife wielder is very possible given the defender has some training. Still, there is some risk of getting cut.
Anecdotes of store clerks or others disarming a knife or gun wielder are not the rule, but lucky exceptions against an uncommitted attacker. A small wild pig can turn away a jaguar if the big cat isn't too hungry or without hungry young.
Unarmed defense against an experienced, committed, knife wielder is crazy, unless the defender is well trained and willing to accept the probability of getting cut. There is a risk of lethality or severe injury so engagement should be a last resort.
Low grappling stances are good for grappling, higher upright stances are good for striking/kicking. Sometimes one doesn't know the exact nature of the threat so any combat stance taken must be balanced (both for usage and weight distribution) to allow for fast reaction, distance adjustment, angle changes, launching and receiving strong attacks.
MMA with BJJ is very effective in combat due to the broad skill set, hard contact training and PT done by the serious practitioner. Boxing is very good, too, but with a more narrow skill set as technique is quite constrained by rules.
Sport karate develops many combat skills, but has some rule constraints making it limited in grappling/close-in situations. Pre/non-sport karate has no rules and a different skill set, including close-in fighting.
All MA have something to offer. Not all people want, or are able, to be professional level fighters, take numerous hits, or commit to many hours/wk training and conditioning, but still desire to have some combat oriented physical activity. People are not just apples and oranges, but a whole fruit salad including mangos, bananas, kiwi, pears, and so on.
I am bored with the constant comparisons and evaluations of one art with another. Let's just agree that they all have benefits for each person according to their needs, wants and abilities.