Don't use the word "guaranteed" to talk about neutrinos from GZK.  Although they are strongly expected to be there, nothing is guaranteeed in science and if they were guaranteed it wouldn't be interesting to discover them.  The cutoff in the CR spectrum could be due to the sources turning off at that energy.  Who knows Lorentz Invariance could be violated and make them all brem electrons before they reach us.

BC: By extension, don't refer to the GZK cutoff as proof of neutrinos needing to exist. It's purely circumstancial (at this point) that CRs are supressed right at the cliff of the proton+photon -> delta threshold.