Touratech is currently showing a weight-optimized conversion of a BMW 1200 GS Rambler for off-road use. In addition to reducing the weight, the fork was also replaced and one from the BMW F800 GS Adventure was installed with a 2.15 x 21 inch spoke rim. This provides much more guidance in curves than the standard front wheel, which collapses more quickly on loose surfaces.
On the road, the driver doesn't want a smaller diameter for handling purposes.
Finding the right compromise is the big challenge for manufacturers, almost all of whom offer a travel enduro, but with different concepts.
The 2017 Honda Africa Twin is equipped with a 21" front wheel.
Ducati Multistrada 1200 Enduro has 19" like the Multistrada 950
Ducati Multistrada 1200, however, only 17"
Suzuki V-Strom 650 19"
KTM 1190 Adventure 19"
BMW 1200 GS are also all on 19".
The wish would be like some supermotos with two sets of rims, one for on-road and one for off-road.
So far, the best compromise for heavy enduro tourers seems to be the 19" tires on spoke rims.