Glueball
Curiously, [not laughing now] . . . no winglet retrofits have yet been suggested for the popular "modern day" B777, nor for its latest factory fresh ER and LR versions.
Remember the Triple 7 originally was to come with the option for a folding wingtip but no one wanted it.. added weight and complexity.
And Boeing has joined with Aviation Partners to put winglets on the other machines.. because it is not there now does not mean it won't be there later.