If you never discharged below ~30% or charged above ~80% of their true capacity, and made sure they always stayed within a comparatively narrow thermal envelope

If you never discharged below ~30% or charged above ~80% of their true capacity, and made sure they always stayed within a comparatively narrow thermal envelope, your batteries would achieve the same lifespan. The last quarter-charge in either direction is where ~80% of the lifecycle wear comes in, and temperature excursions account for much of the rest.