What is perhaps missing from this discussion is the usual advantage of IS or VR with zooms, which is sometimes missing with primes, unless one uses a sensor-stabilized camera, as Erik does.
Since most of my shots are taken without tripod, image stabilization is important for me. I'd be very reluctant to buy a 400mm prime without IS or VR, even if it were unusually sharp, lightweight and low cost. The Nikkor 85/F1.8 doesn't boast VR, and that's a disadvantage, but not so serious a disadvantage as would be the case with a longer focal length.
Taking some more shots in bright sunlight today, comparing the prime with the zoom, this disadvantage became apparent. A 1/125th with the zoom at F11 and ISO 100 would have been sufficient, but not with the prime. To compare resolution fairly, I used ISO 400 and 1/500th for both shots, hand-held.
Now, what's interesting is that even at F11 the prime seems a tad sharper, at least to my 70-year-old eyes when viewing a 100% crop on my HD monitor.
Of course, such differences are subtle and probably only of interest to connoisseurs of fine resolution.