Couplers/Rohloff/Belt are luxuries. Emergency stops are not. I guess I'm not the only guy to investigate discs for touring etc since we already have the term "mullet" for bikes with front disc/rear rim brake. & it was decades ago that I read in Bicycling Magazine about tandem tourers that insisted on a rear drum brake. Seems that if tandems regularly exceed the capacity of rim brakes then singles might be pushing the limits also.