Your point is valid to an extent, but I personally think this is only true for some routes. Some will travel from other airports, some won't travel at all. If there's enough peioke wanting to fly from SNN, its good business to fly them. I think a Monday/Friday schedule would have made the route very popular.
There's also the point that if there was a Monday/Friday schedule, it would steal a lot of PAX from the ORK service.