Maximality conjecture for the moduli orbifold

For $g\geq 3$, let $\M_g$ be the smooth moduli orbifold of genus-$g$ Riemann surfaces. Maximality conjecture for $\M_g$. The orbifold $\M_g$ does not finitely orbifold-cover any other smooth orbifold. This would rule out proper finite orbifold quotients arising from larger discrete isometry groups; the source gives no resolution.