The diagram in the answer by the OP does not conserve lepton number.
Conservation of lepton number is one of the experimental facts which gave rise to accepting the standard model , SM, as the basic model of particle physics .
It is known that there exist experimental facts which are not within the SM and need an extension of it, for example CP violation in the model is not enough to explain cosmological data. Neutrino oscillations are not included in the SM because there their masses are assumed to be zero and an extension of the SM is needed.
A neutrino oscillation correct Feynman diagram, within a reasonable extension ( consistent with the rest of the data) of the standard model will allow for the reaction to happen , and is this one :
neutrino oscillation is indicated by the cross. For the OP's channel reduce the number of vertices by eliminating the e+e- pair.
There are also other diagrams possible in Grand Unified theories, or Super symmetries that allow for the possibility of the channel under discussion .
When looking for extensions of the SM one has to embed it, i.e. follow the main structure , because the SM is an encapsulation of an enormous amount of data, and any ad hoc additions will most probably violate something that is supported by a lot of data. If , as in the proposed by the OP diagram , Z exchanges could go with lepton number violation, a lot of unusual channels would have opened at LEP, which were not seen. There would not have been a lepton number rule and there would have been a different standard model.
Note that the diagram above still depresses the probability at low energies for the decay channel, as the OP diagram, but at LEP energies a lot of violations of lepton numer would have been seen if the diagram of the OP were correct..