Also, in the beginning when you got a non-winglet aircraft compared with a winglet-aircraft, the difference would not only be the winglet itself, but also a reinforced wing (Stiffer wing). The combo of winglet/stiffer wing gave a big advantage in fuel burn etc regarding the "softer" wing without winglets. But now every 737NG is being delivered with the wing already made stiffer. Adding the winglet makes a smaller difference now, because the stiffer wing also reduces fuel burn. So the economics have changed in that regard as well. Add the short routes etc, I think you have now a complete picture. (Someone told me this when visiting the factories at Seattle... I didn't invent this story, so don't blame me if can't reproduce it 100% correctly...
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GrTz B.D.