In some cases. This happened in the EU during the COVID pandemic due to weird regulatory reasons.

But in general: if no one is booked on the plane, they will stop selling that flight pretty quick. Of course in the short term they might fly an empty flight if all passengers no-showed and if they need the plane at the other airport, but their fleet allocation optimizer probably works at most a couple days ahead, so empty flights will soon be culled. Similarly routes with few passengers will get allocated smaller planes in the future, etc, etc.

Airlines often also cancel flights due to lack of demand, sometimes at the last moment, because rescheduling the passengers is more profitable than flying the plane.