Ted my man, you are being highly speculative and hyperbolic. If it so happen if I am sick I just don't fight. Or is it you meant T3H R34L LIFE (TM) self defense. If I am old or just infirmed, it will be unlikely I will on the street shouting to my "theoretical opponent", " Come on mate, this grandpa is gonna show you whose ur daddy!"
Now to give you a hyperbolic answer: Should such scenario ever occur to me, you will bet I would have considered:
a) egressing from the confrontation, failing which...
b) I will ask for help, assuming none are available, then...
c) use tools to equalize the confrontation, if I so happen to be in a barren desert with no appropriate tools... then
d) I will give a fight for my opponent to remember me by...
Nobody ever goes looking for confrontations. Those things just occur to the unwary. Criminals are streetwise. They will only attack if they feel they have an advantage. There will be two or more people working together. The attacker will have a weapon ready. Even if you had an RPG slung over your back, it will take time to ready it.
Budo was created by homicidal, paranoid samurai. They understood that the thread of life was fragile. It could be cut down in the middle of a heart beat. Therefore they trained to be aware 24x7. Much of this training was of the mind as well as body. One could even say that they used the body to get to the mind.
Of course I hope a confrontation like this does not happen to you. I hope it doesn't happen to any of us. However I think the training of being aware all the time is a valuable legacy. It is a goal that we can all try to achieve in our practice.
The Yoshinkan that I practice also stress on the factors I highlighted. But we speak very little about ki stuff. Maybe that was just your perspective.
A friend of mine studied Tai Chi with a Chinese instructor for 11 years. During that entire period, the teacher only mentioned Chi once. Many instructors believe that when the student is ready then they will understand. So there really isn't any need to mention it.