Originally Posted by
ben m
The proton is a composite particle containing three quarks, and we know what holds these quarks together. The quarks are fundamental. No known theory or experiment requires any substructure "held together" inside a quark. Electrons are also fundamental. No known theory or experiment requires any substructure "held together" inside an electron. That answers the question as far as I'm concerned, until someone points out a problem in these details, which you have (still) not done.
One problem is that in proton-antiproton annihilation we never see quarks. Another is that the lack of electron substructure does not account for the Einstein-de Haas effect or magnetic dipole moment. Another problem is that we can create so-called fundamental" particles via say pair production, but what you call "known theory" does not explain the mechanism involved. The overriding problem is that people like you are essentially
fundamentalists.