Bill, I can tell you why the clearances on the tourers are as they are.
Most people won't have any other shifters except Shimano STIs.
STIs work well with cantilever or mini-vee brakes, but most people believe that cantilevers are rubbish.
So, I have to design a bike that works with mini-vee brakes
Picture https://www.flickr.com/photos/52358536@N06/14930320539/in/album-72157624571269648/ shows a 35mm Marathon tyre with 10mm mudguard clearance....the guard is as high as it will go, and the guard is touching the mini-vee wire. (That's an original Marathon, the new Greenguard ones are taller)
With the guard as low as it will go https://www.flickr.com/photos/52358536@N06/14930321849/in/album-72157624571269648/ its pretty close to a 32mm tyre, but there are people who want to run 25 or 28mm tyres without it looking too "gappy". (pictures are 700c bikes, but the limitations of mini-vee brakes are universal.)
If I'm going to be riding tracks, or even minor lanes in the winter, I find that much less than 10mm mudguard clearance will cause the guards to jam up with mud.
Spa list Marathon Supremes, Racers, Greenguard in 26 x 1.5 or 1.6. The Racers and Supremes are nice light tyres for road use or dry tracks, but they will "let go" in anything resembling mud. My favourite tyre for roughstuff was the original Marathon, with a Kevlar strip; good on the road, and surprisingly capable in anything but the foulest mud. The new "Greenguard" tyres are much heavier and give a dull ride due to the thickness of rubber under the tread.
I can't second-guess the bike trade; I have laid up a lifetime supply of 8 and 9 speed transmission parts quite needlessly, because I can still buy them; I should have hoarded tyres.....
