Who in the world buys large primes?
Wildlife and sports photographers, mostly.
I'm struggling to see how one would find use in a long focal length prime, 300mm and above for example: without zoom, isn't your shot composition always at the mercy of how close or far away your subject is, meaning heavy cropping is almost always necessary in post?
It's the difference between cropping and heavy cropping. Would you prefer ending up with a 6MP image? Or a 12-15MP image?
When shooting distant subjects, where you can't get closer, either from issues of personal safety, regulations, or stressing a bird to the point of abandoning a nest of young, if you had a zoom, you'd be zoomed in all the time, anyway.
I own the Canon 400mm f/5.6L USM prime lens. It's lighter than the 100-400L, about half the price (or less) is sharper than the 100-400L Mk I, just as sharp as the Mk II, and autofocuses more quickly. It's also a bit better at taking a teleconverter. And delivers images to me that I simply couldn't get with a 70-300mm zoom lens--especially with cropping. For a birder, feather detail is something you really really like. For a bird-in-flight or sports shooter, responsiveness is huge.
And if you have the big bucks, Canon also makes a 400/2.8L. They don't make 400mm zooms that fast.