Saddle: how good is narrow?
Sit bone (i.e., ischial tuberosity) width is an important consideration, but it is only a starting point, rather than the complete answer.  Your riding position also affects saddle width choice, with more upright postures being better supported by wider saddles.

Too wide only becomes a problem if you get chaffing or have problems getting behind the saddle for technical riding (e.g., cross-country mountain bike riding, without a dropper post - i.e., old school). 

If the B68 is working for you then I wouldn't over think it.  Conversely, I can tell you from personal experience the discomfort of a Brooks saddle that is too narrow.  As the leather breaks in and you "sink" in a bit, you can start hitting the outer metal support frame with your sit bones.  Adding some more leather tension can help, but it doesn't completely solve the issue (I had to sell the saddle).

Why B17 has become the defacto standard is probably partly because it is a good saddle that breaks in quickly, and partly it has to do with the underground lore that evolved around it.  Sometimes its best not to meet our heros.