That (5 cm tether) coincides pretty well with where a tether will out-mass a rigid kite.
I would think for a soft kite, the tether is relatively much heavier. Lightweight, slow moving kite, with relatively large pull… Perhaps for soft kites, tether mass would be a big issue even before 1 MW scale… Would be an interesting analysis to perform… (edit: Im not thinking clearly here - soft kites are probably also cubic scaling in mass. The kite weight will escalate in mass the same speed as the tether)