I don't think this is being done for shaft longevity reasons. Daithi is following Phreaky Phil's lead in putting a rear disc on R100GS(ex-PD). He is using the KI final drive because that is the same generation as the R100GS paralever, & more particularly has the mounts to allow this.
My '91 R100GS thread is here
My Ride photos are here
It's just better when there's twins involved....