Although most Aikido training is based on reactive (go no sen) situations, there is a whole class of techniques based on sen no sen (siezing the initiative).
I can't disagree with the fact that is the first part of your statement there. That's how we train. However-
"O Sensei: In Aikido, there is absolutely no attack. To attack means that the spirit has already lost. We adhere to the principle of absolute non-resistance, that is to say, we do not oppose the attacker. Thus, there is no opponent in Aikido. The victory in Aikido is masakatsu and agatsu; since you win over everything in accordance with the mission of heaven, you possess absolute strength.
(interviewer): Does that mean ~o no sen? (This term refers to a late response to an attack.)
O Sensei: Absolutely not. It is not a question of either sensen no sen or sen no sen..."
I've cut short his answer. I'm not claiming this statement has practical value to me either, just pointing it out... (I think the squiggle there should be a g, yeah?)