The Fuji sensor is 1.25x larger horizontally than 35mm.
These F4 lenses are like F3.2 on 35mm. I certainly don't consider F3.2 fast. In fact you get a shallower DOF with Fuji's APSC cameras/lenses.
So as it stands, I only see this system appealing to people who need resolution at the moment. For the photographers who shoot FF at fast apertures (shallow DOF), this would be a downgrade.
Kei's talking about large format in the case of f/4, which is indeed very fast. There's even the Kodak 178mm Aero Ektar which is f/2.4, which is equivalent to f/0.8 in terms of DOF.
f/2.8 on medium format is shallow enough for most people, and the bigger format makes the DOF fall off differently, I always find that there's a much clearer delineation between sharp and out of focus as you move up formats. The higher acuity and the smoother bokeh you'll get will make things appear shallower than what an online DOF calculator will tell you. Check out
Bryan Schutmaat's work or this photo essay on the New York Times:
http://www.nytimes.com/interactive/2016/09/17/opinion/Occupy-Wall-Street.html?_r=0
Both were shot on 4x5, probably with 150mm f/5.6 lenses which would equate to 50mm f/1.8 on 35mm. Still looks shallower than f1.8 because the bokeh is extremely smooth but where it's in focus it's super sharp. The NYT set probably has most shots around at f/8-f/11 as well. The same thing happens to a lesser degree with medium format, even with the 'cropped' 645 that the Fuji has.
Additionally a larger, squarer format also gives you more natural looking wide angle shots without as much rectilinear distortion compared to 35mm.
So no, this system (and by association digital MF) isn't just for resolution junkies.