I have thought this for years - irimi is direct and tenkan is a waste of time - it is only what happens when irimi fails - and yet, you are already dead yourself. I have often been looked at in disbelief. Great article.
Rupert, maybe you and I are defining tenkan differently - if by tenkan you mean a circular dance I agree with you. But I am a firm proponent of the belief that every tenkan is essentially an irimi.
The example I use is when I see a parking spot open up across the street and I make a fast uturn to grab it before anybody else can. It is a tenkan but it is 100% direct irimi.
So to me, on the mat, my tenkan is a movement FROM irimi in order to deal with something like a tactical need to get my attacker moving in a particular direction due to the presence of obstacles or other attackers.