Graham, of course you are entitled to your own decision and opinions on the subject for sure.
In my experiences dealing with real people and real scenarios that are hell bent on really harming you, I have to say I have found that Ellis' analysis on irimi to be spot on with current military teachings on the subject for both the U.S. Army and the Marine Corps.
To be honest, I am actually fascinated that just about everything I have studied that is founded on good, solid koryu to be as relevant today as it was hundreds of years ago.
Of course, tactics aside as they constantly change and adapt based on the application of technology.
However, and the fundamentals, they are the same.
You can continue to move off line as you wish if you have found that works for you....maybe it is semantics and we would be able to get together and see we are looking at the same thing though using different words/paradigms to describe.
For me though, moving off line does nothing to affect uke, it does not disrupt his movement or motion...it only delays and when you consider how the brain works, looking at the concepts of Hicks law especially you simply do not have in reality much time to make a choice and hope that it is correct. the law of this process of movement dictates that you are not in control of the situation and nage is. that in and of itself means that nage can process movement and choices faster than you. So you may get lucky on occasion and move off line correctly.
The odds are based on the relationship between uke and nage that nage will move more correctly than you can ever move. So you have one chance to change the situation and that is to irimi as Ellis has described. For me it is about Odds and I have found when I move as Ellis has described against a bad guy either in reality or training that I fair much better than moving off the line.
I think is partly semantics. If you want to stay on line and via timing or sen no sen beat the other person to the strike then that's one thing but it's not irimi. The word itself is to do with passing or passing behind in fact.
I could explain irimi from the viewpoint of energy, natural paths of energy thus natural motion, or from geometric viewpoint. However, as soon as someone says you don't have to move off line then I know they are mixing it with something else. Samurai wise I'd call this sacrifice.
Anyway, to see my point clearly all you have to do is think of a bus coming at you or if you like a raging bull. Irimi may save your life but staying on line gets you killed, flattened.
Now we come to the next point inherent in my view. Contrary to what many may think entering off line for me is not to do with evasion or avoidance but purely to do with 'being with' ie:joining.
Now if you do so and as you pointed out above thus allowing the attacker to remain on his course without disruption then the result is not as you describe or envisage. For to him you have 'dissappeared' and yet you are there with him and in control. The surprise it'self takes his center let alone his mind and stability.
In fact sen no sen etc. is all part of entering properly off line and is not at all reactive.
Tai-sabaki by the way for me has nothing to do with irimi either as irimi is entering on a straight line and tai-sabaki is entering on a curve. Moving out in order to cut on the same line is yet another thing, not the same thing.
So basically, so as not to be misunderstood, I am saying irimi, entering off line correctly is meant to not disrupt the opponent for its purpose is to put you in a position where you can now disrupt and cannot be disrupted.