>the resolution of 4x5 film easily exceeds any commercially available digital sensor, with pretty much any halfway decent lens.
Theoretically perhaps, but you have to look at the whole imaging system (including the inevitable loss from scanning or optical printing in the case of film). Even five or ten years ago the best digital systems were able to capture about as much detail as you could realistically squeeze out of a 4x5 negative. If you disagree, I'd be interested to see some comparison shots backing this up. I shoot 4x5 as a hobby, but I've long ago given up any illusion that it has practical resolution benefits over the best digital equipment.
It's also worth mentioning that for realistic large format photography, accurate focus and camera shake start to become significant limiting factors. The actual experience of using a 4x5 camera (outdoors at least) tends to consist of taking your best guess at accurate focus while squinting at a dim ground glass through a loupe, and then hoping that you triggered the shutter at the precise moment when the wind stopped blowing.
Finally, one has to take diffraction into account. Typical apertures for 4x5 landscape photography are around f22-32. A good chunk of the theoretical resolving ability of the film and lens is lost once you're stopped down to that extent.