5.6 is too slow for a prime....... like the 800-5.6?
Prime is about the quality of the optics, the quality of the build, the quality of the sealing..... in short a prime lens is a higher quality lens than a regular lens. Yes, they tend to be faster lenses, but it is not speed that makes it a prime lens, it is quality. Not all prime lenses have to weigh 10 pounds and cost $10,000, there is a market for lenses of various degrees of quality and performance.... look at the 70-200 lenses....
and a 400 f2.8 is hard to hand-hold, take hiking up a mountain, or carry on a canoe trip.....