Why do older airplanes have the wing curved upward like a 787?
You are confusing 2 concepts here, dihedral and winglets. What you are showing in the picture is dihedral, which is not a curvature but an angle positive from the horizontal. This angle helps improve lateral (roll) stability in flight. This is nothing new, almost all commercial jets have dihedral. Winglets improve efficiency. A Boeing 787 has both.