Zed bot would get bullied out by a ranged Marksman and the Marksman's support. Probably.
ADCs are different, and if people started, say, taking Assassins into bot, then ADC mains would change accordingly. Caitlyn, Ashe, and Varus would be valued for their longer ranges. Urgot would be valued for his tankiness and dueling. Vayne would be valued because Vayne.
Then there's towers. You can't dive everybody, and a Marksman gives a team a safe option for shredding towers from a distance, rather than having to get in melee to damage it.