We report the synthesis and melt self-assembly behaviors of densely grafted, core−shell bottlebrush (csBB) polymers derived from covalently linking narrow dispersity, symmetric composition ABA-type triblock polymers through their chain midpoints. Derived from sequential ring-opening polymerizations of εdecalactone and rac-lactide initiated from 5-norbornene-2-exo,3-exo-dimethanol, poly(lactide-block-ε-decalactone-block-lactide) macromonomers (M n = 9.2−17.8 kg/mol; Đ = 1.19−1.25) were enchained by living ring-opening metathesis polymerization (ROMP) into csBBs with backbone degrees of polymerization N bb = 8−43. Temperature-dependent small-angle X-ray scattering (SAXS) studies indicate that the critical triblock arm degree of polymerization (N arm ) required for melt segregation decreases with increasing N bb , leading to reductions in the accessible ordered lamellar microdomain (d) spacings. We derive a phenomenological relationship between the critical triblock arm segregation strength at the order−disorder transition (χN arm ) ODT and N bb to enable the future design of microphase separated core−shell bottlebrushes, which self-assemble at sub-10 nm length scales for nanolithography and nanotemplating applications.