This TNG50 preprint examines Milky Way-Andromeda analogues through kinematic decomposition via the Mordor tool, selecting bulgeless systems with B/D < 0.08 against bulge-dominated controls (B/D > 1). Among the bulgeless subset, 74% experienced at least one major merger yet preserved disks through gas-rich, coplanar, and corotating encounters that delivered angular momentum rather than erasing it. Bulgeless galaxies assemble half their stars at z50 ~ 0.7, later than bulge-dominated counterparts at z50 ~ 1.2, and maintain thin-disk dominance (D/T > 0.5) from z ~ 1 onward with minimal bulge growth after z ~ 1.5. The simulation shows these galaxies occupy lower-mass halos with elevated specific stellar angular momentum, aligning with Milky Way observations, though their specific star-formation rates and gas fractions remain modestly higher than empirical constraints. What the original coverage underplays is the role of early smooth accretion and alignment: even bulgeless galaxies without major mergers display structural differences traceable to filamentary gas inflow that sets favorable spin orientations before z = 2. This echoes findings in the original IllustrisTNG50 papers (Pillepich et al. 2019, MNRAS) on angular-momentum acquisition and complements observational work on Gaia-Enceladus-Sausage and other ancient mergers (Helmi et al. 2018, Nature) that were likely gas-rich enough to rebuild rather than destroy disks. Limitations include TNG50's finite resolution (~10^5 solar masses per baryonic particle), which may under-resolve minor-merger heating, and the reliance on a single simulation suite whose subgrid physics for feedback and star formation remain tunable. As a 2026 arXiv preprint, the study awaits peer review and independent verification with higher-resolution runs such as TNG100 or NewHorizon.