I can understand why you think that, but I still believe that ma ai exists at every point. If we use the irimi example:
Uke attacks with any technique, we irimi into his blind side, my ma ai has decreased significantly to the point that I may actually be in body to body contact. This would still be ma ai for me in order to move to the next part of my technique i.e. I still need to move to continue with a technique, and assuming my ma ai is correct, uke will also need to move again in order to attack again.
Ma ai is usually defined as "correct distance", how about describing it as safe distance (not a literal translation of course)? Many people consider Ma ai to be the starting point, as you mentioned above, the point at which one or other has to take a step to initiate an attack, therefore it can also be considered safe distance. If uke attacks and I do irimi to the blind side, I have moved up close and personal to uke, but, at that point in time it is a safe distance because he couldn't hit me without moving. Just a thought.