as well as it would be cheaper.
That could be a problem: The 400 prime would have to go against the 100-400 in the same bracket, and the 200-400/4 (or 400/2. for the high end customer. Which means a limited customer base to spread development costs around, keeping the unit price higher then that of the current model. Plus the mandatory adjustments of course.
As for wide angle zoom and 15-85, wouldn't that be the standard zoom? It has a quite narrow FOV at its long end after all.