im waiting too...
either for a new 400mm f5.6 with IS or a new 100-400mm.
i really don´t like the current 100-400 and it´s push/pull design.
my father has one and it´s by far the lens that attracts the most dirt.
I have stopped to wait and decidetd to get a 400mm f5.6 NON-IS, because
the 100-400 air pump should be addicted to dust - as you wrote.
Another reason is that the old 400 mm prime has just 6 lens elements and
should show less flare than a zoom with 14 (!) lens groups.
Another reason to buy the old 400mm is that I don't want to pay twice
the money for an IS version which has - maybe - 12 or 15 lens elements too.
In germany only a few dealers have listed the 5.6 400 and I hate to be too
late - after many frustrating decisions!
I observed that classic lenses are perhaps not as sharp as modern zooms (and
IS lenses) but have better contrast and render the object with higher fidelity. So
the percepted authenticity of photographs is better than that of the zooms.
My first impressions with that lens: very sharp, very contrasty, easy for hand-holding,
absolutely sturdy design (one metal tube enclosing/supporting the movable parts) and
a big advantage: the built-in hood with rubberized rim to take some rough bumps (I do
not plan them but they might happen).