Down was the original sofa fill, and nothing invented since has quite replaced it. The reason is not luxury; it is geometry. Down clusters are three-dimensional — a down plume holds its loft by having nothing to settle against — and a down-wrapped cushion recovers its shape after each use because the clusters redistribute rather than compress in place. Foam, by contrast, compresses exactly where it is sat, and nowhere else.
A down-wrapped cushion wants a little care. It should be turned. It should be fluffed, once in a while, the way a bed is made in the morning. In exchange, it stays beautifully for a long time. A cushion in year ten is not the cushion of year one, and that is the point.