You know I am thinking of a 400mm f/5.6L that I will combine with a 1.4X II (or even 2XII) for sunsets, sunrises, ducks, etc.
But my sunrise/sunset lens is a 300mm f/4L (non-IS) with the above mentioned teleconverters.
Also my "duck" lens (yes I have given it this nickname!) is a 100-400L.
So it would be too much. (The reason for fixed 400 is IQ wise I do not wish to use the 2X).
A new 400 (either as a fixed lens or as a 100-400 zoom) would be the best for me (and I guess thousands more...)