Also, the difference between 600-800mm is the same as the difference between 60-80mm.
There's a 60mm for EF-s, but for FF there's practically nothing between 50-85mm, a much bigger gap in percentage terms than the gap from 600-800mm.
Yes, there's zooms that cross that range, ie 24-70/2.8 and 24-105/4, but f/4 between 50-85mm is nowhere near the f/1.2 you can get at those lengths.
There's the Sigmonster zoom that crosses the 600-800mm at f/5.6, and the 800mm prime is f/5.6 too, any 700mm prime would be probably f/5.6 too. So when you think about it, if 601-799mm is the length you need, you're better served than if you need 61-79mm...