My understanding is that for electrons, there is a standard argument that the electromagnetic interaction between them is required, not optional. Since they're identical particles, we should be able to take the wavefunction of two electrons and mix up their identities by any amount we like, and the amount of mixing can vary locally. This is a gauge symmetry. The result of the gauge transformation is that the individual wavefunctions pick up derivatives that don't match up properly with their physical energy and wavelength. To make it work, we have to take the gauge function and make it into a physical field that carries energy and momentum. So if we didn't already know about the electromagnetic field, we'd be forced to invent it. But what about neutral fermions? Some examples would be neutrons (which I guess you could object are composites of charged fundamental fermions), neutrinos (which you could say do participate in electroweak interactions), and sterile neutrinos (what's their excuse, assuming they exist?). It seems too good to be true that we could look at a neutral fermion like the neutron and infer that it's composite. And I don't see what fails in the original argument just because the fermion we're talking about is composite. I dont' speak field theory fluently, so I'd appreciate any attempts to dumb down answers to the level at which I presented the question.