I don't think it's good business model that a premium full service carrier will allow a passenger to pay a "premium", then move him/her to a low-cost carrier for part of the journey where even a bottle of water costs money. So the higher yield comes from tricking the customer on false expectations?

The most successful low-cost carriers out there don't typically facilitate connecting traffic even if both are on their metal. So it's all O&D traffic from their perspective.