they’ll just go work somewhere else.

This implies there are 1) similar employers 2) in a similar location, and 3) that are looking for workers, which isn't a guarantee.

And even if there was, that puts the burden of looking for a new job on the workers. Why should it be their responsibility? We don't have to get to this part of the conversation in the first place: In N Out shouldn't be banning masks in the first place, regardless of what contingencies are available or unavailable to their employees.