In my experience being able to stay on the line of attack and do irimi is neither a question of sen no sen. Nor is it a sacrifice. It is just a question of technical skills or possibilities. And a question of how you understand aikido in a wider sense.
Cutting through the attacker instead of letting him by are just different "images" of aikido, I think.
This is a video of the swordwork we do.
You may notice that shidachi doesn't move off line. But he cut's through the sword of uchidachi anyway.
This can be seen as a kind of paradigm of the tai jutsu we practice.
Completely different setting but identical scheme when entering yokomen uchi.
Irimi on the line of attack.
shomen uchi ikkyo omote.
Please notice that tori here also doesn't leave the line.
O.K. Nice video. I don't call this irimi. I see no irimi there. Entering and staying on line yes. You must be 'tuned in' to do this effectively thus it is in the end a matter of sen no sen etc. As in the o/p it can't be done as a reaction.
If in your Aikido you class this as part of irimi then fair enough, thus I may understand how some may be using the term so thanks for that. However, for me I would never use irimi to mean staying on line.