"ADCs are AD carries - they don't need nerfs, they're supposed to carry"

Well there are other ways around that. Range and fluidity (Juggernauts), skillshots and zone control (Mages), and high risk for the reward (Slayers) are all ways the other combat classes of the game are capable of carrying outside of just having power spikes.

Granted, none of these other variables matter for Marksmen so Marksmen really should have some kind of down-time gating, but it's entirely possible for a carry to carry without just being in the right stage of the game while still having weaknesses to compensate. Again, Marksman don't have these other weaknesses, but carries as a whole can and do.