Comparison of uncertainties of binned distance moduli between ∼3000 simulated volume-limited z < 0.3 LSST SNe with (green) and without (orange) DESI follow-up, and Union3 (blue). We plot the square roots of the diagonal elements of the distance modulus covariance matrix, and σ_corr corresponds to the first off-diagonal. Without spectroscopic follow-up, the new statistical power of LSST SNe can hardly be utilized as systematics approach values comparable to Union3. The photo-z-induced uncertainties are estimated based on a systematic σ_z = 0.005.