I may be misunderstanding your point ... but IP/aiki really is THE important thing. It is why Takeda, Ueshiba, Sagawa, Horikawa, etc all talked about it. They didn't equate techniques with it, didn't equate fighting with it, didn't equate jujutsu with it, etc. While those other things are important and are things that people can learn, it was IP/aiki that was the well-spring. It was why Ueshiba said, "We would do it this way with aiki" -- because aiki changed how the body functioned, whether in a compliant or non-compliant world.
Perhaps for many, they don't grasp those things... but, for many, now training IP/aiki, they do. And that is an important point to consider ...
IP/aiki provides an edge but it is not by any means complete on its own. Training for fighting requires other attributes as well, such as cardiovascular conditioning as well as raw strength. Both of those feed into and support the combative mindset, since without proper conditioning you gas out. As Vince Lombardi said, fatigue makes cowards of us all.
Layered on that is strategy for particular engagement parameters. As Kevin Leavitt said, it's about controls and parameters.
Let's take a look at this from a sport fighting point of view: judo is different from muay thai, which is different from sanda. To be really good at one, you have to optimize your training program. Even things that seem similar, like muay thai and sanda actually have different rules and therefore different strategies for victory. Last year I witnessed a match between two heavyweights under sanda rules. One of the fighters had fought on a K1 undercard. He lost. Different rules (parameters), and he fought a guy with a lot of experience in that ruleset. The fighters weren't allowed to throw knees, and it looked like his inside/clinch strategy was dependent on that. I should add that I have personally lost a match because of failure to adapt to a particular ruleset, and lulling myself into a false sense of confidence by improperly structuring my training.
Saying that IP/IT is THE important thing misses out on the fact that there are a lot of things that go into making a successful fighter. A person could have all the IS/IP they want and still get knocked out by a straight right hand if they don't know how to move their head or cover appropriately.
Maybe you say that sports isn't your thing and that you're training for the street or "real combat" whatever that is. Well still, the question is parameters. Training with impact, bladed or firearms? What are your goals?
Another thing to remember is that we know a lot more about certain types of physical training today, versus Ueshiba's time. Things like high intensity cardio and periodized strength training were not too well understood then. When Draeger brought modern weight training techniques to Japan, it revolutionized judo, as one of his weight training students said after he won a major competition.
As far as high intensity interval training, Tabata's work didn't rise to prominence until the 1990s. That stuff counts. I watched a couple of videos of Jigen Ryu and to me, it looks like they accidentally stumbled onto high intensity interval training with weapons. Looks silly right? Simplistic? Simplistic is good when it comes to training big groups of people.
I would bet that the reason they were feared is because they were in good shape to keep hitting hard even after exertion.
Look at MMA-- there are people with incredible levels of conditioning, and it's not just about "working hard" because everyone is working hard. It's about working smart-- eating the right diet, having the right periodized training, the right mix of skills etc.
My personal experience is that internal training can give an edge, but only in the context of 1) understanding the engagement parameters and 2) having the full suite of proper athletic conditioning in addition to IS/IP.