The number of fermion generations has many effects beyond the mere existence of "3 copies of fermions," including determining scatterings, constraining CP violation, influencing electroweak symmetry breaking, contributing to anomalous magnetic moments, and influencing baryogenesis.
How would the universe differ if there were only 1 generation of fermions, assuming no other standard model couplings changed? (Meaning the electron, 1-neutrino, up, & down Yukawa couplings; the CKM & PMNS phases; $g_1$, $g_2$, & $g_3$; the QCD vacuum angle; and the Higgs $\lambda$ & $\mu$ stay the same.)
Specifically, how would observed particle properties and interaction amplitudes change? And how would those changes affect baryon asymmetry and nucleosynthesis?