So why a zoom, and why 2.8? Given the sort of shots they're almost always used for, does anybody really use UWs at their long end? And does anybody really need them to be 2.8? In my DSLR days I used the (lovely) Sigma 10-20 f/4-5.6. But shot almost everything around 10mm and f/8. And I think most people I know who use UWs do pretty much the same. So, why not develop a nice, compact 10mm f/4?
I would disagree as different types of photography often lead to different ways of shooting. I used focal length of my DSLR UW zooms but I was often shooting in environments where I had little to no control or might need to reframe from one position. Shooting wide and cropping in can change the "feel" of a shot or slow down a workflow on a tight deadline.
The UW is a foundational to the photojournalists tool kit for shooting environmental portraits, action in tight spaces, the "Hail Mary" postgame handshake, crowds and for remote camera setups. Landscape shooters may not be able to get closer or further back.