Hobie engineers please take note
True...a metal set screw in plastic threads pushing on an immovable steel part causing permanent strain in a thinned area of plastic under cyclic loading... I suspect it will not age well. A screw in double shear is usually a preferred connection. Maybe they were worried about corrosion of small parts, parts count, or the need for another tool (a #1 small cross point screwdriver).
This fix may have been easier than replacing the spine. I just punched the fin shaft out, drilled, then hammered it back in. Did not mess with the cables or the idler pulley. The process I feared was drilling through the stainless steel shaft. All I had was a hand drill (no press). Turned out, with a little oil and a sharp bit, it only took a few minutes of drilling.
Another tid-bit: The drive started clicking when the set screw came loose. I doubt it was the sound of the loose set screw tapping the flat on the shaft - I usually blame clicking on the cable to gear teeth alignment - but the drive does seem to operate more smoothly now. If I were a Hobie engineer I would consider purposefully loosening the set screw a turn or two and see if it causes clicking. That clicking sound is so annoying.