In ΛCDM, high-resolution simulations predict that Milky Way–mass halos should host hundreds of dark matter subhalos massive enough to form stars, yet only a few dozen luminous satellite galaxies are observed around the Milky Way and Andromeda, the classic “missing satellites” problem (Klypin et al. 1999; Moore et al. 1999). Although survey incompleteness, reionization, and feedback can hide or darken some subhalos, matching both the total number and internal kinematics of the brightest satellites without overproducing galaxies in the most massive subhalos—the related “too big to fail” issue—remains a nontrivial challenge for standard ΛCDM prescriptions (Boylan-Kolchin et al. 2011; Bullock & Boylan-Kolchin 2017).