Re: aikido vs jiu-jitsu
I understand what you meant, and I agree. My point is this. You don't always have that luxury, especially at full speed.
There are two ways to find out. 1. a real fight. 2. replicating conditions as close as possible to a real fight.
Real fights can take many forms. from a testertone fight in a bar, to a real fight in which someone intends to inflict serious harm.
You can practice at full speed in a dojo within parameters. I personally have found more often than not that irmi tenkan or moving off the line does not always work, sometimes you mess up your timing and uke is able to "roll over you" and take your balance.
I don't disagree with you, I just caution against taking a cavalier stance of "just simply move off the line". It is not always an option.
I believe if your focus is outside of the "DO", and you want to prepare for all sides of empty hand fighting, then you need to address the full spectrum of fighting from stand up, closing the distance, takedowns, and ground fighting.
I have found, for myself as well, that it is easy to establish a paradigm within our own art of what works and doesn't work. It is a false sense of security. When we train only with those in our art it is easy to be lured into the perspective that "moving off the line" works everytime. Just need to be congnizant of the this when you train.
I recommend going to some local grappling schools in your area and mixing it up with them for an eye opener in moving off the line.
Again, it is correct, but my point is...does not always work for you!