Invalidated very simply - attacker is standing motionless, but his position is preventing me from reaching a desired goal. For example, assisting a friend who is being assaulted by the attacker's compatriot.
Also it assumes that anything moving is, but it's very nature, unstable. That is not, as Gary pointed out, always the case.
This is the kind of thing that I heard a lot of when I first started Aikido - seems to make sense, but upon examination you find that there are just too many situations where it just doesn't hold up.
Good points. It is why I prefer OODA as a model for a dealing with a threat as it does not necessarily require physical movement or contact. To be successful in winning under the theory of OODA you simple need to take whatever action is necessary to disrupt the decision cycle of your opponent. Thinking in terms of Musahsi it can be as simple as meeting your opponent on the beach with the sun behind your back.
If we deal with things purely in terms of the immediate and physical, we lose sight of a great deal of what is really important which is the strategic. IMO, this is what we are striving to reach in our studies, an keen understanding of cause and effect and being able to affect things long before the physical.