Some of the points made by the OP trouble me also. I have intended to replace my 18mm with 16mm since it appeared on the roadmap.
I already have the 18mm. It and the 35mm are my most used lenses (I have the 18, 27, 35 & 56). While the 18mm's corner sharpness is inadequate for landscape/architecture and it's distortion is inadequate for architecture, one of the things I like about the xf18 is f/2 provides enough light to document debauchery at parties or out on the town (and the lens is small and unobtrusive enough for this purpose). Now the specs of the 16mm are out, I wonder whether I would carry the 16mm in the same situations as I do the 18mm. If I did, I would gain a faster auto focus, a handy focus clutch and the ability to crop from 16 to 18mm at f/2-2.8 with no perceivable difference in sharpness given the much higher resolution of the lens. Alternatively, I buy the 14mm and keep the 18mm.
However, I don't think I would ever carry more than one wide angle prime when far away from home.