I'm a Canon EF 400mm F5.6L user. Although the design is old, but it still perform very well and IMHO, if the price is going to increase for an upgrade, I would prefer Canon leave it as it is.
Super telephoto lenses are expensive and not everybody can afford $2K++ (I'm one of them) for a lenses. So for those who are interested in wildlife or bird photography, but had limited $$, IMHO, this is basically the most suitable lenses out there in the market... fast AF, sharp, light weight and very cheap (compare to other super telephoto lenses).
Anyway, if you look at the pricing of super telephoto lenses, there is a big jump in pricing after EF 400mm F5.6L... may be Canon can put a new super telephoto lenses (eg. a non-DO 400mm F4L IS) at that price level for those who can afford a bit more especially if EF 300mm F2.8L IS is going to be discontinoued.
Have a nice day.