So, just for arguments sake, the master and vats are untouchable.
Even in this scenario, humans seem to multiply much more quickly than SMs. Humans also have ingenuity and intellect, something which most SMs do not have (minus vault raids).
Problem is, SMs, while raiding for 'prime normals', would take casualties as well. One would argue the raid would makeup for those lost. But what about in large scale standing battles where the SMs are fighting to take ground that is strategically important but devoid of towns or vaults. How about casualties through guerilla skirmishes, in these fights there are no guarantees the human fighters are prime candidates, or even if there were, enough to replace losses.
Lastly, SMs do not seem to have the ability to operate normie sized vehicles. They don't seem to have real transport of any kind besides specially constructed wagons for dipees or the wounded. This is compared to varying groups of normies with artillery pieces, airpower, very few but workable vehicles, etc. Even if they were somehow successful, they would lose the logistical battle in the end run.