I realized the aperture would impose that sort of constraint. I was more uncertain about the length. If the length could be reduced significantly, that might be worthwhile, at least to some people (maybe not enough).At 400mm the size of the lens is 95% defined by the aperture and not the sensor size. A 400mm 5.6 lens needs a 71mm diameter front element, no matter the sensor size. And all other lens element need to be sized accordingly.
Smaller aperture, like 6.3 and less corrections would make a lens smaller but dropping 1 stop would be too much. Don't think anyone wants a 400mm F8 lens.